Refactors in preparation to support SOP in File System

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 69.31%. Comparing base (dc50836) to head (b1d95df). Report is 3 commits behind head on master.

Files	Patch %	Lines
btree/btree_with_transaction.go	44.44%	0 Missing and 5 partials :warning:
btree/btree_node.go	33.33%	0 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #68 +/- ## ========================================== - Coverage 71.38% 69.31% -2.07% ========================================== Files 5 5 Lines 1541 1659 +118 ========================================== + Hits 1100 1150 +50 - Misses 319 370 +51 - Partials 122 139 +17 ```

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