Streaming snapshot checkpoint in Tsavorite

[badrishc]

StreamingSnapshot is a new checkpoint type that yields a stream of (mostly but not all distinct) key-value pairs that correspond to a consistent snapshot (version v) of the database. This can be used to create a checkpoint of the user's preferred format, or directly streamed to replicas, etc. To recover, the user needs to simply iterate over this stream in order, performing a sequence of upserts, and this will restore the consistent v version (state) of the database.

Treating this iterator as a checkpoint allows users to pair it with AOF replay to get exactly once processing semantics (TBD).

Current implementation makes the mutable region temporarily read-only, similar to a snapshot checkpoint. However, this can be optimized to avoid the copy-on-write in future, by making the update threads write out the old version v before updating it to v+1 in place.