hide more file writing details from engine

[tabokie]

This is a preparing work for #258.

Changes:

• Move fsync to file handle instead of writer, so that we can call it concurrently in the future. In doing so, we have to remove the sync offset tracking and deprecate bytes_per_sync support
• Move prepare_write into pipe_log. Introduce a trait ReactiveBytes for this purpose.
• Rotate the file during write instead of doing it every write group, the timing is also delayed to the next write after exceeding limit (instead of the write that exceeds the limit).
• Differentiate two types of I/O errors: unrecoverable error from fsync, and retriable error from pwrite. Only bubble error for the latter case, panic early for unrecoverable ones.
• Also refactor the purge code, fix two cases where force-compact is not triggered

[tabokie]

/cc @LykxSassinator

[codecov[bot]]

Codecov Report

Base: 97.65% // Head: 97.66% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (3e15000) compared to base (9751c6d). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #269 +/- ## ========================================== + Coverage 97.65% 97.66% +0.01% ========================================== Files 30 30 Lines 10645 10633 -12 ========================================== - Hits 10395 10385 -10 + Misses 250 248 -2 ``` | [Impacted Files](https://codecov.io/gh/tikv/raft-engine/pull/269?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | Coverage Δ | | |---|---|---| | [src/env/mod.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2Vudi9tb2QucnM=) | `100.00% <ø> (ø)` | | | [src/env/obfuscated.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2Vudi9vYmZ1c2NhdGVkLnJz) | `95.65% <ø> (-0.19%)` | :arrow_down: | | [src/config.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2NvbmZpZy5ycw==) | `96.66% <100.00%> (-0.03%)` | :arrow_down: | | [src/engine.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2VuZ2luZS5ycw==) | `97.90% <100.00%> (-0.01%)` | :arrow_down: | | [src/env/default.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2Vudi9kZWZhdWx0LnJz) | `92.26% <100.00%> (-0.13%)` | :arrow_down: | | [src/file\_pipe\_log/log\_file.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2ZpbGVfcGlwZV9sb2cvbG9nX2ZpbGUucnM=) | `100.00% <100.00%> (ø)` | | | [src/file\_pipe\_log/pipe.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2ZpbGVfcGlwZV9sb2cvcGlwZS5ycw==) | `99.49% <100.00%> (+0.49%)` | :arrow_up: | | [src/log\_batch.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2xvZ19iYXRjaC5ycw==) | `97.97% <100.00%> (+<0.01%)` | :arrow_up: | | [src/memtable.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL21lbXRhYmxlLnJz) | `99.16% <100.00%> (-0.01%)` | :arrow_down: | | [src/pipe\_log.rs](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL3BpcGVfbG9nLnJz) | `93.93% <100.00%> (+0.28%)` | :arrow_up: | | ... and [6 more](https://codecov.io/gh/tikv/raft-engine/pull/269/diff?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv)

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[tabokie]

Ping @Connor1996 , this one is blocking #261