Configurable astLedgerSequence offset, finite default value for max_fee.

set default value of max_fee to 10000 (0.01xrp).  "infinity" is less suitable for default value, 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.