report on txn pool fullness

[brianolson]

Summary

Add report line about txn pool fullness.
txnpool({lowest reading}, {min(node mean txn pool size)}, {mean(node mean txnpool size)}, {max(node mean txnpool size)}, {highest reading})

For a test running across 20 nodes, each node has a node mean txn pool size across a 20 minute test time. The above line reports the min/avg/max of those txn pool average sizes, and also the lowest and highest txn pool size seen at any moment.

Test Plan

This is a script used in development testing.