use option<> for purge_threshold and recovery_threads

[tonyxuqqi]

This is to make it possible for the caller to dynamically choose the default value under different situations like single rocksdb or multi-rocksdb.

[codecov[bot]]

Codecov Report

Patch coverage: 98.03% and project coverage change: -0.01 :warning:

Comparison is base (3c442af) 97.89% compared to head (b90d51b) 97.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #299 +/- ## ========================================== - Coverage 97.89% 97.88% -0.01% ========================================== Files 31 31 Lines 12188 12199 +11 ========================================== + Hits 11931 11941 +10 - Misses 257 258 +1 ``` | [Impacted Files](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | Coverage Δ | | |---|---|---| | [src/config.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2NvbmZpZy5ycw==) | `97.22% <95.23%> (-0.42%)` | :arrow_down: | | [src/engine.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2VuZ2luZS5ycw==) | `98.63% <100.00%> (ø)` | | | [src/file\_pipe\_log/pipe.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2ZpbGVfcGlwZV9sb2cvcGlwZS5ycw==) | `98.09% <100.00%> (ø)` | | | [src/file\_pipe\_log/pipe\_builder.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL2ZpbGVfcGlwZV9sb2cvcGlwZV9idWlsZGVyLnJz) | `95.97% <100.00%> (ø)` | | | [src/purge.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-c3JjL3B1cmdlLnJz) | `97.45% <100.00%> (ø)` | | | [tests/failpoints/test\_engine.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-dGVzdHMvZmFpbHBvaW50cy90ZXN0X2VuZ2luZS5ycw==) | `99.89% <100.00%> (ø)` | | | [tests/failpoints/test\_io\_error.rs](https://codecov.io/gh/tikv/raft-engine/pull/299?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#diff-dGVzdHMvZmFpbHBvaW50cy90ZXN0X2lvX2Vycm9yLnJz) | `100.00% <100.00%> (ø)` | | 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]

Seems unneeded.