refactor: add engine txn API

[bsbds]

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?)

  First PR of the entire engine refactor, it adds StorageOps and TransactionApi trait.

• 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 91.74312% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 75.69%. Comparing base (296105f) to head (4acd008). Report is 72 commits behind head on master.

Files	Patch %	Lines
crates/engine/src/rocksdb_engine/mod.rs	88.09%	0 Missing and 5 partials :warning:
crates/engine/src/api/operation.rs	90.00%	1 Missing :warning:
crates/engine/src/memory_engine/transaction.rs	85.71%	1 Missing :warning:
crates/engine/src/proxy.rs	96.77%	1 Missing :warning:
crates/engine/src/rocksdb_engine/transaction.rs	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #674 +/- ## ========================================== + Coverage 74.52% 75.69% +1.17% ========================================== Files 171 182 +11 Lines 24949 27015 +2066 Branches 24949 27015 +2066 ========================================== + Hits 18593 20450 +1857 - Misses 5206 5348 +142 - Partials 1150 1217 +67 ```

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