Detach database management from partition manager

For #6101 (closed)

In Gitaly, the partition manager (a part of WAL) manages a key-value database. This database is backed by badger. When the partition manager starts, it initializes the database for each storage. It also starts garbage collection goroutine. A database is closed when its counterpart storage finishes.

In the future, when we integrate Raft into Gitaly, it will properly need access to the underlying key-value database. This MR detaches the database management out of partition to the existing keyvalue package. The new manager initializes, starts GC goroutines, and provides access to the ready databases.