Integrate partition assignment logic into partition manager

This commit plugs in the partition assignment logic into partition
manager. Instead of looking up a TransactionManager by the relative
path, the PartitionManager gets the repository's assigned partition
and routes the transaction to the correct partition based on that.

The partition state directories are now also identified by the
partition ID rather than the relative path as each partition may
contain multiple repositories soon.

While this change makes partition manager compatible with multiple
repositories in the same partition, the TransactionManager still
doesn't support multiple repositories in a partition. Support will
be added later.