Ketch: Basic replication system

Ketch: Basic replication system

Git Ketch is a multi-master Git repository management system.  Writes
are successful only if a majority of participant servers agree.  Acked
writes are durable against server failures as a majority of the
participants store all required objects.

Git Ketch is modeled on the Raft Consensus Algorithm[1].  A ketch
sailing vessel is faster and more nimble than a raft.  It can also
carry more source codes.

Git Ketch front-loads replication costs, which vaguely resembles a
ketch sailing vessel's distinguishing feature of the main mast on the
front of the ship.

[1] https://raft.github.io/
Change-Id: Ib378dab068961fc7de624cd96030266660b64fb4