rpc/pool: use urkel proofs for namestate in spv mode

[pinheadmz]

Closes #646
Also see kyokan/bob-wallet#409

Restores SPV node RPC functionality for getnameinfo getnameresource verifymessagewithname and getnamebyhash.

Each of these rpcs now accepts an extra boolean parameter safe. When true, the node gets historical data from the "safe height" which is the last tree root commitment with more than 12 confirmations. This is obviously historical data that any full node will have locally, but is also data that can be retrieved from the p2p network by an SPV node. This is the exact same mechanism used by SPV (and hnsd) for light client name resolution.

Example:

Full Node

# get current info
hsd-rpc getnameinfo chaos

# get info committed at last tree interval
hsd-rpc getnameinfo chaos true

SPV Node

# ALWAYS NULL
hsd-rpc getnameinfo chaos

# get info committed at last tree interval (gets proof from p2p network)
hsd-rpc getnameinfo chaos true

Here's the tradeoffs for using "safe height":
👍 SPV nodes can getnameinfo and getnameresource without relying on a centralized API
👎 This data may be outdated since it can only be verified with the most recent urkel tree root commitment
👎 Getting Urkel proofs from the p2p network may be slow and the RPC may time out

[coveralls]

Coverage increased (+0.1%) to 67.28% when pulling 2170fe5 on pinheadmz:spv-namestate into a747dc8 on handshake-org:master.

[pcfreak30]

This is working fine for my needs. I acknowledge the risks of cached data from this.