In this commit, unify the old and new values for `sync-freelist`, and
also ensure that we don't break behavior for any users that're using the
_old_ value.
To do this, we first rename what was `--db.bolt.no-sync-freelist`, to
`--db.bolt.sync-freelist`. This gets rid of the negation on the config
level, and lets us override that value if the user is specifying the
legacy config option.
In the future, we'll deprecate the old config option, in favor of the
new DB scoped option.
This commit separates all etcd related sources (sans a few stubs and
config) from the rest of the source tree and makes compilation conditional
depending on whether the kvdb_etcd build tag is specified.
