Structured Logs: follow up - replace cond_wake with level-triggered flag

[JoshuaMoelans]

For context #1271 (comment)

Since we changed the logs batching to always re-trigger a flush upon addition, we can take one step further and eliminate the use of condvars, replacing them with a level-triggered atomic flag. This flag would also replace the current retrigger in the flush loop (reintroducing an explicit producer trigger). The flusher would then not sleep to wait, but use a futex-wait or WaitOnAddress to maintain a timing-based schedule in addition to the producer-side level trigger.

Part of #1398

[linear]

NATIVE-130