PebbleDB integration to fulfill `KeyValueDB`

[timwu20]

Issue Summary

Taken from design doc:

db.Backend is constructed using an implementation of the database.Database interface. We have already introduced the DBAdapter type which takes an underlying kvdb.KeyValueDB implementation and implements database.Database. We will have to implement an implementation of kvdb.KeyValueDB that is powered by PebbleDB and use that when constructing Client and it's associated db.Backend.

Acceptance Criteria

• Implementation of kvdb.KeyValueDB that utilizes PebbleDB
• Unit tests
• Utilize in constructor for db.Backend as DatabaseSource.