Refactor manager internal API, speed up string filters in UI, add manager API tests

[sc1f]

This PR reduces the public API/code footprint of PerspectiveManager by moving all internal logic to a separate base class, which should make the Manager API more "manageable" and easier to read. Additionally, it reduces the amount of computation required for string filters by making the suggestions list quicker to compute and serialize.

Python Changelog

• Add PerspectiveManagerInternal, which hosts all internal methods that maintain the wire API.
• Add tests around PerspectiveSession being able to open/close multiple sessions
• Add tests around PerspectiveManager handling on_update and remove_update over the wire API
• __ROW_PATH__ values are now python values that match the underlying schema (datetime.datetime for "datetime" columns, etc.) instead of strings. This behavior is now consistent with perspective.js.
• Makes _PerspectiveCallbackCache easier to use by removing the need to get the underlying list before operating on its contents
• Changes _PerspectiveCallbackCache.remove_callback to remove the callbacks for which the lambda returns True. This changes its behavior and makes it no longer consistent with filterInPlace, which is used in the Javascript version of Perspective to remove callbacks.
• Removes numpy.isnat and fixes several pandas and numpy tests around datetimes
• Replaces isinstance(str) with isinstance(six.string_types) for Python 2 compatibility

JS Changelog

• Makes filter suggestions for string columns faster by a magnitude of ~10x (on large datasets)
  • Because we don't use the actual columns of the dataset to generate the suggestion, make the new view with columns=[] instead of serializing all columns and immediately disposing of them.

[sc1f]

@texodus this branch is rebased on top of #1082 - please merge that PR first so we have consistent line endings, otherwise the eol=lf in .gitattributes is preventing me from most git operations because line endings are automatically changed, even if I discard those changes in the working directory

[texodus]

Looks good! Nice catch on the autocomplete performance bug!