log: cache recent threads up to a day

log: cache recent threads up to a day

In general, a pthread implementation reuses thread ids routinely so the
circular_buffer is there to help us see what the thread name was historically
(capped at 4 entries).

However, to guard against this map from growing without bound, discard entries
that are more than a day old. This would happen if a thread logged an Entry and
has since disappeared for more than a day.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>
(cherry picked from commit bfe489ed19d599b67cacb0bdd4958f36661248f2)