Replace O(N) lookup with O(1) for task outputs.

This speeds up suite validation significantly for tasks with
large numbers of outputs.  The list used required an O(N)
check for membership which, when combined with being run for
each output resulted in O(N**2) complexity.

Using the builtin set should bypass any deprecated libraries in
Python 2.

Closes cylc#3255

cylc/flow/config.py

@@ -1590,8 +1590,7 @@ def generate_taskdefs(self, orig_expr, left_nodes, right, seq):
 
             # Record custom message outputs.
             for item in self.cfg['runtime'][name]['outputs'].items():
-                if item not in taskdef.outputs:
-                    taskdef.outputs.append(item)
+                taskdef.outputs.add(item)
 
     def generate_triggers(self, lexpression, left_nodes, right, seq,
                           suicide, task_triggers):

cylc/flow/taskdef.py

@@ -64,7 +64,7 @@ def __init__(self, name, rtcfg, run_mode, start_point, spawn_ahead):
         self.expiration_offset = None
         self.namespace_hierarchy = []
         self.dependencies = {}
-        self.outputs = []
+        self.outputs = set()
         self.param_var = {}
         self.external_triggers = []
         self.xtrig_labels = {}  # {sequence: [labels]}