Redesign of the attestation pool (#14324)

* Rename existing AttestationCache

* Cache with simple Add

* fix import cycle

* logic for unaggregated, aggregated and block attestations

* some small fixes

* remove Seen

* finishing touches

* feature flag

* extract forkchoice atts to separate type

* gate new functionality behind feature flag

* revert test files

* preparing for review

* change Id to [32]byte

* Potuz's review

* Potuz's review pt 2

* Nishant's review

* keep flat list of atts

* fix ForkchoiceAttestations() function

* Tests for Add, GetAll, Count

* Tests for remaining functions

* use DeepEqual

* fix tests

* documentation

* changelog <3

* v2 handlers

* nil check for forkchoice atts

* guard against 0 bits set

* fix failing test

* Preston's review

* better godocs

config/features/config.go

@@ -49,6 +49,7 @@ type Flags struct {
 	EnableHistoricalSpaceRepresentation bool // EnableHistoricalSpaceRepresentation enables the saving of registry validators in separate buckets to save space
 	EnableBeaconRESTApi                 bool // EnableBeaconRESTApi enables experimental usage of the beacon REST API by the validator when querying a beacon node
 	DisableCommitteeAwarePacking        bool // DisableCommitteeAwarePacking changes the attestation packing algorithm to one that is not aware of attesting committees.
+	EnableExperimentalAttestationPool   bool // EnableExperimentalAttestationPool enables an experimental attestation pool design.
 	// Logging related toggles.
 	DisableGRPCConnectionLogs bool // Disables logging when a new grpc client has connected.
 	EnableFullSSZDataLogging  bool // Enables logging for full ssz data on rejected gossip messages
@@ -267,6 +268,10 @@ func ConfigureBeaconChain(ctx *cli.Context) error {
 		logEnabled(EnableDiscoveryReboot)
 		cfg.EnableDiscoveryReboot = true
 	}
+	if ctx.IsSet(enableExperimentalAttestationPool.Name) {
+		logEnabled(enableExperimentalAttestationPool)
+		cfg.EnableExperimentalAttestationPool = true
+	}
 
 	cfg.AggregateIntervals = [3]time.Duration{aggregateFirstInterval.Value, aggregateSecondInterval.Value, aggregateThirdInterval.Value}
 	Init(cfg)