Although the dlg session count was incremented in a pjsip servant
thread, there's no guarantee that the last thread to unref this
progress object was one.  Before we decrement, we need to make
sure that this is either a servant thread or that we push the
decrement to a serializer that is one.

Because pjsip_dlg_dec_session requires the dialog lock, we don't
want to wait on the task to complete if we had to push it to a
serializer.

Change-Id: I8ff2d5d94be3ff04298394070434e22a7d3cbc41