set finite value for default max_fee,  and configurable offset for lastLedgerSequence.

set default value of max_fee to 10000 (0.01xrp).
infinity is a less suitable as it might cause unexpected high fee for
users who unawared of it.

Replace the arbitrary number of 8 with a configurable
"lastLedgerOffset".
Emit 'error' when lastLedgerSequence was over, so that a transaction is
error_handled and cleared from Queue.