Rename log_sink to log_write_func to make it more clear.
Remember the list of log targets as static member LogTarget.all_targets.
Make each LogTarget instance register with the list of targets implicitly.
No longer create a default log target, rather create one explicitly in
osmo-gsm-tester.py.
Change-Id: I5844485eaed536cb34b44bfe23dc635fe1982dcd