core: metrics collection for transaction events (#3157)

* 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