initial snapshot state machine implementation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.53799% with 41 lines in your changes missing coverage. Please review.

Project coverage is 70.91%. Comparing base (6633bd8) to head (ee07596). Report is 34 commits behind head on master.

Files	Patch %	Lines
src/raft/recv_install_snapshot.c	82.75%	29 Missing and 11 partials :warning:
test/raft/unit/test_snapshot.c	98.96%	1 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (6633bd8) and HEAD (ee07596). Click for more details.

HEAD has 4 uploads less than BASE

| Flag | BASE (6633bd8) | HEAD (ee07596) | |------|------|------| ||6|2|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #659 +/- ## ========================================== - Coverage 77.41% 70.91% -6.50% ========================================== Files 196 196 Lines 27269 26664 -605 Branches 5455 2833 -2622 ========================================== - Hits 21111 18910 -2201 - Misses 4297 5419 +1122 - Partials 1861 2335 +474 ```

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