refactor: xline storage api

[bsbds]

Depends-On: #673 #806

Adds an XlineStorageOps trait as a replacement to StorageApi which is removed in #806

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.82560% with 115 lines in your changes missing coverage. Please review.

Project coverage is 76.03%. Comparing base (e35b35a) to head (73bb8ab). Report is 117 commits behind head on master.

Files	Patch %	Lines
crates/xline/src/storage/db.rs	72.81%	21 Missing and 7 partials :warning:
crates/engine/src/rocksdb_engine/transaction.rs	83.70%	14 Missing and 8 partials :warning:
crates/engine/src/proxy.rs	90.71%	17 Missing :warning:
crates/engine/src/rocksdb_engine/mod.rs	90.36%	3 Missing and 5 partials :warning:
crates/engine/src/memory_engine/mod.rs	89.06%	5 Missing and 2 partials :warning:
crates/xline/src/server/command.rs	64.70%	3 Missing and 3 partials :warning:
crates/xline/src/server/maintenance.rs	62.50%	1 Missing and 5 partials :warning:
crates/curp/src/server/storage/db.rs	0.00%	0 Missing and 5 partials :warning:
crates/engine/src/memory_engine/transaction.rs	0.00%	4 Missing :warning:
crates/xline/src/storage/kv_store.rs	42.85%	0 Missing and 4 partials :warning:
... and 4 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #853 +/- ## ========================================== + Coverage 75.55% 76.03% +0.48% ========================================== Files 180 187 +7 Lines 26938 28155 +1217 Branches 26938 28155 +1217 ========================================== + Hits 20353 21409 +1056 - Misses 5366 5436 +70 - Partials 1219 1310 +91 ```

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