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.
The `lost` and `missing` semantics [don't have anything to do](https://github.com/ripple/ripple-lib/blob/be78db1563f8a07c171d6ef666ea7f661374fecd/src/js/ripple/transaction.js#L59-L60) with `LastLedgerSequence`, but I don't object to making this change, as it gives these events some foundation in reality. Not a fan of your switch though.

Also,  a transaction could be resubmitted with a different fee. Unless you call `tx.lastLedger(sequence)`, the `LastLedgerSequence` will be automatically bumped on every resubmission. I'm not a big fan of this, but I can't accept your pull request without further consideration, as it would break things.