raft: add tests for progress.maybeDecr
etcdhttp: return 404 when removing nonexistent member
Merge pull request #1492 from jonboulle/1492_nonexistent_member

Attempting to remove nonexistent member mishandled
Merge pull request #1522 from jonboulle/raft_tests

raft: add tests for progress.maybeDecr
raft: minor cleanup in comments