We do a linear scan of all pubkeys for each validator and slot - this
becomes expensive with large validator counts.
* normalise BN/VC validator startup logging
* fix crash when host cannot be resolved while adding remote validator
* silence repeated log spam for unknown validators
* print pubkey/index/activation mapping on startup/validator
identification
* fix REST liveness endpoint responding even when gossip is not enabled
* fix VC exit code on doppelganger hit
* fix activation epoch not being updated correctly on long deposit
queues
* fix activation epoch being set incorrectly when updating validator
* move most implementation logic to `validator_pool`, add tests
* ensure consistent logging between VC and BN
* add docs