Datastore backed Logstore

Work pretty much inspired by go-libp2p-peerstore great implementation.

This is a go-datastore backed implementation of go-threads/core/logstore:

• AddrBook
• HeadBook
• KeyBook

For testing, two go-datastore implementation are table-tested:

• badger
• leveldb

Tests leverage the /test suites used also for in-memory implementation.