Calculate average transactions per second

Use OnTxAccepted notifications to track the count of transactions that get
accepted in the memory pool. Also save the time of the first and last
transactions in order to calculate the average transactions per second by
dividing transaction count to the difference of time between first and
last transaction.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.