* in exit pool, filter out already-packaged messages; bundle remaining messages into beaconblocks
* filter messages at block construction time
* allow adding up to intended capacity of buffers, beyond per-block limits
* document rationale/design for filtering mechanism
* skeleton of attester slashing pool & validators
* add skeleton for proposer slashings and voluntary exits; rename pool to more inclusive exit pool to stay consistent with all three; ensure is initialized by beacon_node so is safe to merge, even if it doesn't do much yet
tersec