This commit adds the compaction feature of the bbolt compact to our bolt
backend. This will try to open the DB in read-only mode and create a
compacted copy in a temporary file. Once the compaction was successful,
the temporary file and the old source file are swapped atomically.
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.
