Refactor cluster interfaces

Instead of all methods lumped into BulletinBoard, inter-node
communication is split into separate Master and Slave interfaces.

MasterServer and SlaveServer implement these as appropriate.

I kind of think I wrote ourselves into a corner if/when we do
master election, but there's not that much code, so we can
refactor that later.

Will look into multicast discovery for RMI, a la elasticsearch,
which would be totally cool.

Thinking about ditching JDBI/H2 since we technically don't need to
keep any persistent state.