libtracker-miner: Keep GTimer objects around forever
libtracker-miner: Keep GTimer objects around forever

Logic to create and destroy on demand was slightly broken, and for the
extra complexity we were only saving 48 bytes of memory.

Fixes GB#676437.

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