Fix snapshot monitoring library to not cancel monitoring invalid snapshot.

Fix snapshot monitoring library to not cancel monitoring invalid snapshot.

snapshot DSO unregistered itself when snapshot changed state to invalid.

This can cause a race (and several timeouts), when for example another snapshot
is added and in the middle of operation (suspend/resume) the monitoring thread
unregister itself.

Fix it by keeping the snapshot monitored after invalidation - just reset
threshold to not really print any messages to syslog.