Reduce ETX expiration age

Since we take a snapshot of the EtxSet after every block, the longer an
ETX stays in the EtxSet, the more its storage requirements compound.
Since we do not yet have pruning/gc for EtxSets, we need to reduce the
ETX expiration age for now. One day, pruning will obviate the need for
this.

core/types/etx_set.go

@@ -5,7 +5,7 @@ import (
 )
 
 const (
-	EtxExpirationAge = 8640 // With 10s blocks, ETX expire after ~24hrs
+	EtxExpirationAge = 100 // With 10s blocks, ETX expire after ~24hrs
 )
 
 // The EtxSet maps an ETX hash to the ETX and block number in which it became available.