Use g_task_set_source_tag on all of our async methods

This is supposed to help with debugging and profiling per the docs for
g_task_set_name, which is automatically called by g_task_set_source_tag in
glib >= 2.76.0 which is our minimum requirement.

Testing Done:
Consulted with the turtles

Reviewed at https://reviews.imfreedom.org/r/2955/