Exposes flush calls.
Adds safety, at the cost of a panic risk, as multiple TXNs simultaneously
writing to a key will now cause a panic. This should be fine and the safety is
appreciated.
zstd was recommended for the base layer only, due to its CPU requirements. That
was a misreading on mhy behalf.
lz4 gets ~5% better compression than snappy with ~30% faster performance. zstd
does ~25% better than lz4 yet at ~30% of the performance.
