Update whitespace and exports formatting
Reorder functions into a logical progression

This just moves functions around in the mem3_rep module to give a better
logical progression. Purely stylistic but it should make things easier
to read and find.
Add a new mem3_rpc module for replication RPCs

This is intended to make the local/remote code execution contexts a lot
more clear.
Inline open_doc_revs into open_docs

This function was trivial and never reused. It was more confusing to
have it as a separate function rather than just inlining into where it's
used.
Include replication history on checkpoint docs

This changes how and what we store on internal replication checkpoint
documents. The two major changes are that we are now identifying
checkpoint documents by the database UUIDs (instead of the node that
hosted them) and we're storing a history of checkpoint information to
allow us to be able to replace dead shards.

The history is a list of checkpoint entries stored with exponentially
decreasing granularity. This allows us to store ~30 checkpoints covering
ranges into the billions of update sequences which means we won't need
to worry about truncations or other issues for the time being.

There's also a new mem3_rep:find_source_seq/4 helper function that will
find a local update_seq replacement provided information for a remote
shard copy. This logic is a bit subtle and should be reused rather than
reimplemented.