Review synchronizer, rearange some stuff, add notes and TODOs.

db/l2db/l2db.go

@@ -17,6 +17,8 @@ import (
 	"github.com/russross/meddler"
 )
 
+// TODO(Edu): Check DB consistency while there's concurrent use from Coordinator/TxSelector & API
+
 // L2DB stores L2 txs and authorization registers received by the coordinator and keeps them until they are no longer relevant
 // due to them being forged or invalid after a safety period
 type L2DB struct {