Introduces custom types in protobuf to serde directly into multiaddrs
and peer IDs. Simplify purge by ordering addrs by expiry. In general,
getting this readier for merge.
Each entry in the address book KV store now represents a full peer
with all its addresses. Expiries and TTLs are kept inline. Records are
serialised with Protobuf.
Housekeeping is performed on retrieve, and via a periodic GC routine.
The architecture mimics a write-through cache, although not strictly.
