Logging: implement get_filter_names and small fixes.

get_filter_names(id: ID) : set[string] returns the names of the current
list of filters for a specified log stream.

Furthermore this commit makes a number of logging functions more robust
by checking existence of values before trying to modify them. This
commit also really implements (and tests) the enable_stream function.

testing/btest/Baseline/scripts.base.frameworks.logging.remove/.stdout

@@ -0,0 +1,7 @@
+{
+default,
+f1
+}
+{
+
+}