More optimizations and clarifying comments

Compared to the previous commit:

| Benchmark          | 36_threadpool_opt_PR | 36_threadpool_opt_cond10    |
+--------------------+----------------------+-----------------------------+
| imap_unordered_seq | 1.06 ms              | 1.02 ms: 1.04x faster (-4%) |
| imap_unordered_par | 965 us               | 928 us: 1.04x faster (-4%)  |
| imap_seq           | 1.08 ms              | 1.03 ms: 1.04x faster (-4%) |
| map_seq            | 785 us               | 870 us: 1.11x slower (+11%) |
| map_par            | 656 us               | 675 us: 1.03x slower (+3%)  |
| apply              | 1.14 ms              | 1.12 ms: 1.02x faster (-2%) |

benchmarks/bench_threadpool.py

 # -*- coding: utf-8 -*-
 """
-Benchmarks for thread locals.
+Benchmarks for thread pool.
 
 """
 from __future__ import absolute_import

src/gevent/queue.py

@@ -597,7 +597,7 @@ class Channel(object):
             self.getters.remove(waiter)
             raise
         finally:
-            timeout.cancel()
+            timeout.close()
 
     def get_nowait(self):
         return self.get(False)

src/greentest/test__pool.py

@@ -375,13 +375,15 @@ class TestPool(greentest.TestCase): # pylint:disable=too-many-public-methods
         it = self.pool.imap_unordered(sqr_random_sleep, range(SMALL_RANGE))
         self.assertEqual(sorted(it), list(map(squared, range(SMALL_RANGE))))
 
-    def test_empty(self):
+    def test_empty_imap_unordered(self):
         it = self.pool.imap_unordered(sqr, [])
         self.assertEqual(list(it), [])
 
+    def test_empty_imap(self):
         it = self.pool.imap(sqr, [])
         self.assertEqual(list(it), [])
 
+    def test_empty_map(self):
         self.assertEqual(self.pool.map(sqr, []), [])
 
     def test_terminate(self):