snapuserd: Add an API call to wait for device deletion.

This adds a new message to the daemon protocol, which waits for a device
to be deleted. The caller must ensure that the corresponding control
device is actually going away (eg, the device containing the dm-user
table entry has been deleted). Otherwise, this will hang.

This will allow libsnapshot to safely delete the cow since any
outstanding references will be closed.

This also refactors DmUserHandler so that it's freed (and removed from
the handler list) if its corresponding thread exits of its own accord.

Bug: 168554689
Test: vts_libsnapshot_test
Change-Id: I8e97c543eec84874c88795a493470e992dc476fc