client: invalidate mount info cache when a mount is removed

If another mount later appears at the same location, a gvfs client
process would use the cached mount info from the previous mount, so
would attempt to talk to the old mount process. Client processes
with an open connection to the mount process would invalidate the
cache properly when that connection closed.
1 job for mount-info-cache in 34 seconds (queued for 2 seconds)
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