Allow disabling stream memoization

[philippjfr]

Addresses #1215, allowing disabling memoization when a specific stream is active, which forces a callback to be triggered when a new event comes in.

[jlstevens]

Having already given you all my feedback in person, I am happy to merge as soon as there are unit tests checking the memoization is working as we expect.

[jlstevens]

That said, there is a sorting issue on Python 3:

Traceback (most recent call last):
  File "/home/travis/build/ioam/holoviews/tests/testdynamic.py", line 176, in test_dynamic_overlay_memoization
    overlay = overlaid[()]
  File "/home/travis/build/ioam/holoviews/holoviews/core/spaces.py", line 758, in __getitem__
    val = self._execute_callback(*tuple_key)
  File "/home/travis/build/ioam/holoviews/holoviews/core/spaces.py", line 617, in _execute_callback
    retval = self.callback(*args, **dict(flattened))
  File "/home/travis/build/ioam/holoviews/holoviews/core/spaces.py", line 443, in __call__
    streams = sorted({s for i in inputs for s in get_nested_streams(i)})
TypeError: unorderable types: PositionXY() < PositionXY()

You could either go back to the old version or just use the original ordering while removing duplicates:

streams = []
for stream in [s for i in inputs for s in get_nested_streams(i)]:
   if stream not in streams: streams.append(stream)

We might have a list deduplication utility already and it might be a good idea if we don't.

[jlstevens]

Looks good. I'll wait for the tests to pass, have one last look and probably merge.

[jlstevens]

Tests are passing and the new stuff looks fine. Merging.