tracker-store: Allow running queries while update is running
tracker-store: Allow running queries while update is running

This will break order guarantees. Check with client applications and
libraries before merging this commit.

outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker