Separate coinc.xml file writing logic from gracedb upload logic.

Currently, both the coinc.xml file writing process and the gracedb uploading process is encapsulated under the __do_gracedb_alert() function in postcoh_finalsink.py.

There is no reason these functions should be grouped together, especially because so much of our research and development happens offline on OzStar. Rather than polluting the error logs and wasting time executing attempts to upload to GraceDb when it is already set as not available, we should simply skip the attempt to upload. To do this, we need to split up __do_gracedb_alert into two functions, and then only trigger the upload step when a client is available.

This should be helpful preliminary work for moving our trigger control to a Redis-based solution in the future.