tracker-db-manager: wrap thread/iface hashtable access with a mutex.
tracker-db-manager: wrap thread/iface hashtable access with a mutex.

Although unlikely, an interface for a thread could be created at the
same time another thread is being cancelled.

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