Snapshot compress

[MathieuBordere]

This PR introduces snapshot compression with lz4. Next to compressing the snapshot, there's an additional data integrity check when decompressing the compressed snapshot that in practice should allow help to detect storage corruptions.

Nodes running compression are compatible with nodes running without compression, data over the wire hasn't changed, this should deal with issues where cluster members are only updated one at a time. The compressed snapshot files are regular lz4 files (without the .lz4 extension though) that can be decompressed with the lz4 cli tool.

Fixes #198

@stgraber If this is merged it will probably break some lxd builds as there will be a new dependency liblz4-dev.

[codecov-commenter]

Codecov Report

Merging #207 (8d45e60) into master (34b3ae0) will decrease coverage by 0.15%.
The diff coverage is 78.75%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   87.26%   87.10%   -0.16%     
==========================================
  Files          47       48       +1     
  Lines        7646     7804     +158     
  Branches     2044     2107      +63     
==========================================
+ Hits         6672     6798     +126     
- Misses        880      900      +20     
- Partials       94      106      +12     

Impacted Files	Coverage Δ
src/uv_fs.c	76.31% <ø> (ø)
src/compress.c	75.37% <75.37%> (ø)
src/uv_snapshot.c	77.72% <95.23%> (+0.83%)	⬆️
src/uv.c	87.40% <100.00%> (+0.43%)	⬆️
src/client.c	78.82% <0.00%> (ø)
src/membership.c	78.64% <0.00%> (ø)
src/replication.c	80.11% <0.00%> (ø)
src/byte.c	99.28% <0.00%> (+0.71%)	⬆️

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