log-walker: Treat the internal state as a critical section
log-walker: Treat the internal state as a critical section

Since the TplLogWalker API is asynchronous, we do not want multiple
overlapping calls to stamp on each others' toes.

Fixes: https://bugs.freedesktop.org/41772

freedesktop-unofficial-mirror/telepathy__telepathy-logger,freedesktop-unofficial-mirror/telepathy__telepathy-logger,freedesktop-unofficial-mirror/telepathy__telepathy-logger