refactor: Fix implicit value conversion in formatPingTime

Summary:

This changes CNodeStats to handle ping timestamps as their original
incoming usec int64_t values until the time they need to be displayed.

Backport of core PR18260.

This fix an implicit conversion warning from Clang.

Test Plan:
With clang as a compiler:

ninja check

Check the warning is gone.

./src/qt/bitcoin-qt

Under the debug console => peer table, check the ping times look
correct.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6277