Refactor core/server DB layer

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.98%. Comparing base (4c5a80b) to head (1fbb46a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #50 +/- ## ======================================== Coverage 99.98% 99.98% ======================================== Files 363 364 +1 Lines 11174 11414 +240 Branches 927 901 -26 ======================================== + Hits 11172 11412 +240 Misses 2 2 ``` | [Flag](https://app.codecov.io/gh/sndnv/stasis/pull/50/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Angel+Sanadinov) | Coverage Δ | | |---|---|---| | [python](https://app.codecov.io/gh/sndnv/stasis/pull/50/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Angel+Sanadinov) | `99.86% <ø> (ø)` | | | [scala](https://app.codecov.io/gh/sndnv/stasis/pull/50/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Angel+Sanadinov) | `100.00% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Angel+Sanadinov#carryforward-flags-in-the-pull-request-comment) to find out more.

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