`ReqIdrToTxn` does not store information about the ledger

Currently ReqIdrToTxn stores a map of sha256(identifier||req_id) -> txn seq_no. It should store which ledger the seq no belongs to, this can be done by changing the map to sha256(identifier||req_id) -> ledger_id<delimiter>txn seq_no

It also makes sense to take into account requests' digest, not only reqId.

We also need a migration script for seqNoDb.