Snapshot pool integration, first version

[letFunny]

This PR integrates the pool with the snapshot installation. This is a first version without the real implementation for the callbacks and I/O. It solves a few issues found along the way, namely it includes:

• pool integration for background work,
• emulating the async nature of rpc sending by scheduling a callback using libuv,
• first versions of rpc_fill and is_a_trigger, that means that the code can now generate primitive messages and expect the correct ones,
• tests now assert that the messages sent are the correct ones,
• added missing state which I noticed when writing new tests.

See the commit list for more information as the functionality is split into self-contained units.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.23237% with 38 lines in your changes missing coverage. Please review.

Project coverage is 77.87%. Comparing base (054b511) to head (defc3f3). Report is 12 commits behind head on master.

Files	Patch %	Lines
src/raft/recv_install_snapshot.c	70.00%	8 Missing and 22 partials :warning:
src/lib/threadpool.c	50.00%	4 Missing and 2 partials :warning:
test/raft/unit/test_snapshot.c	98.44%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #673 +/- ## ========================================== + Coverage 77.84% 77.87% +0.02% ========================================== Files 197 197 Lines 27761 27953 +192 Branches 5493 5529 +36 ========================================== + Hits 21611 21767 +156 - Misses 4333 4346 +13 - Partials 1817 1840 +23 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.