Add "Last Block" and "Last Tx" rows to peer details area

Summary:

qt: add RPCConsole::TimeDurationField helper, call systime only once

gui: add "Last Tx" (CNodeStats::nLastTXTime) to peer details

gui: add "Last Block" (CNodeStats::nLastBlockTime) to peer details

This is a backport of core-gui#226
Depends on D10963

Test Plan:
ninja && src/qt/bitcoin-qt

Select a peer in the "peers" view, check the new "Last Block/Tx" fields. Wait for a while to see it change from "Never" to a duration.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10964