* core: Add metrics collection for transaction events; replace/discard for pending and future queues, as well as invalid transactions
* core: change namespace for txpool metrics
* core: define more metrics (not yet used)
* core: implement more tx metrics for when transactions are dropped
* core: minor formatting tweeks (will squash later)
* core: remove superfluous meter, fix missing pending nofunds
* core, metrics: switch txpool meters to counters
