SDK instrumentation handlers do not flush events

Currently we do not create replay events after receiving new updates from core SDK (e.g. breadcrumbs) -- they only get flushed after an rrweb event.

This causes breadcrumbs to be attached to a new session instead of the previous session.

e.g. old session -> breadcrumbs -> <idle> (no upload happens)
new session -> rrweb checkout -> upload (with old breadcrumbs)
