Ensure we stable-sort transactions in the tx pool

[davidmurdoch]

If two identical transactions are in the pool it may be possible to switch the order in which they are "mined" depending on how many times the pool is sorted.

We may need to sort by a secondary key, the from address may work without adding any additional properties to the transaction -- though this would bias transaction ordering based on the address' lexicographic sort order -- unless we hash the address first, which seems like unnecessary extra work ... or we could used an FIFO-based sort index for this.

[davidmurdoch]

closing in favor of #1104