Decrease number of background threads

Because pythons GIL ☺

src/master/collect-master.py

@@ -32,7 +32,10 @@ import activity
 import importlib
 import os
 
-reactor.suggestThreadPoolSize(4) # Too much seems to have trouble with locking :-(
+# If we have too many background threads, the GIL slows down the
+# main thread and cleants start dropping because we are not able
+# to keep up with pings.
+reactor.suggestThreadPoolSize(3)
 severity = master_config.get('log_severity')
 if severity == 'TRACE':
 	severity = log_extra.TRACE_LEVEL