* restore doppelganger check on connectivity loss
https://github.com/status-im/nimbus-eth2/pull/4398 introduced a
regression in functionality where doppelganger detection would not be
rerun during connectivity loss. This PR reintroduces this check and
makes some adjustments to the implementation to simplify the code flow
for both BN and VC.
* track when check was last performed for each validator (to deal with
late-added validators)
* track when we performed a doppel-detectable activity (attesting) so as
to avoid false positives
* remove nodeStart special case (this should be treated the same as
adding a validator dynamically just after startup)
* allow sync committee duties in doppelganger period
* don't trigger doppelganger when registering duties
* fix crash when expected index response is missing
* fix missing slashingSafe propagation
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
Jacek Sieka