tracker-writeback: Apply writeback handlers sequentially on a same file
tracker-writeback: Apply writeback handlers sequentially on a same file

Activating different writeback handlers on the same file in different threads
may bring in issues, and as the GDBusMethodInvocation is shared across the
(possibly multiple) spawned threads, there's a race condition to have it
handled.

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