Only use head if it's compatible with target (#15965)

* Only use head if it's compatible with target

* Allow blocks from the previous epoch to be viable for checkpoints

* Add feature flag to make it configurable

* fix tests

* @satushh's review

* Manu's nit

* Use fields in logs

config/features/config.go

@@ -69,6 +69,7 @@ type Flags struct {
 
 	DisableResourceManager     bool // Disables running the node with libp2p's resource manager.
 	DisableStakinContractCheck bool // Disables check for deposit contract when proposing blocks
+	DisableLastEpochTargets    bool // Disables processing of states for attestations to old blocks.
 
 	EnableVerboseSigVerification bool // EnableVerboseSigVerification specifies whether to verify individual signature if batch verification fails
 
@@ -274,11 +275,14 @@ func ConfigureBeaconChain(ctx *cli.Context) error {
 		logEnabled(forceHeadFlag)
 		cfg.ForceHead = ctx.String(forceHeadFlag.Name)
 	}
-
 	if ctx.IsSet(blacklistRoots.Name) {
 		logEnabled(blacklistRoots)
 		cfg.BlacklistedRoots = parseBlacklistedRoots(ctx.StringSlice(blacklistRoots.Name))
 	}
+	if ctx.IsSet(disableLastEpochTargets.Name) {
+		logEnabled(disableLastEpochTargets)
+		cfg.DisableLastEpochTargets = true
+	}
 
 	cfg.AggregateIntervals = [3]time.Duration{aggregateFirstInterval.Value, aggregateSecondInterval.Value, aggregateThirdInterval.Value}
 	Init(cfg)