Extend statedb and use prefixes, add debugapi

In statedb:
- Store all values using prefixes for keys to allow iteration
- Add methods MTGetRoot, GetAccounts

Implement debugapi, an http server with debugging endpoints:
- debugAPI.GET("sdb/batchnum", a.handleCurrentBatch)
- debugAPI.GET("sdb/mtroot", a.handleMTRoot)
- debugAPI.GET("sdb/accounts", a.handleAccounts)
- debugAPI.GET("sdb/accounts/:Idx", a.handleAccount)

db/statedb/txprocessors.go

@@ -14,7 +14,7 @@ import (
 
 var (
 	// keyidx is used as key in the db to store the current Idx
-	keyidx = []byte("idx")
+	keyidx = []byte("k:idx")
 )
 
 func (s *StateDB) resetZKInputs() {