libtracker-sparql: Shutdown backends when finalizing connection
libtracker-sparql: Shutdown backends when finalizing connection

This fixes a reinitialization issue when closing and reopening the
connection.

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