[HOLD] collectors: add StateCollector

[djkazic]

Today we don't track LND startup time. This modification records a start and end time for state transitions from RPC_ACTIVE to SERVER_ACTIVE (the bulk of startup time), then makes a prometheus gauge metric which will get picked up on the next scrape.

Edit: it would probably be helpful to make lndmon itself state-aware and not try to do any other scrapes until we see SERVER_ACTIVE.