Fix nits in memory driver

[brandond]

1. We don't need compact_rev_key, thats only used for SQL; the memory driver has m.compactRevision
2. Move broadcast() into appendEntry() since its always called after it
3. Simplify log trimming in Compact, and ensure that compact always leaves one revision in the log for consistency with etcd and sql-based backends.
4. Move a couple strings into constants, since they are common to all drivers
5. Don't allow compacting future or already-compacted revisions, for consistency with etcd