Updates to EC design spec

Mainly around the PUT and reconstructor section topics:

- multi-phase phase commit conversation concept
- reconstructor key concepts
- local data file cleanup
- rebalance and handoff reversion
- misc typo fixes
- additional info on GET handling of prior partial PUTs

Change-Id: I602dcd5f445bf5642c0bc15a6dd30dd40591d4ae
Implements: blueprint swift-ec
Merge "Updates to EC design spec"