Cleanup

This has a few organizational commits, including beginning moving things to the new vocabulary.

Notable changes in vocabulary:

Quorum: 'swarm' was a bad name, because it can mean other things. So the base unit of then network has been renamed to 'quorum', because it's a set of common.QUORUMSIZE units trying to reach consensus. 

Block: a collection of heartbeats that move the network from one state to the next
Sector: a logical collection of bytes on the Sia network. Sectors compose the data once the redundancy is gone.
Ring: a physical collection of data. After a sector gets erasure coded, it becomes a ring. Rings have common.QUORUMSIZE pieces.
Segment: One piece of a ring.