Parallel table proxy

[JSKenyon]

• [ ] Tests added / passed

  $ py.test -v -s daskms/tests

  If the pep8 tests fail, the quickest way to correct this is to run autopep8 and then flake8 and pycodestyle to fix the remaining issues.

  $ pip install -U autopep8 flake8 pycodestyle
  $ autopep8 -r -i daskms
  $ flake8 daskms
  $ pycodestyle daskms

• [ ] Fully documented, including HISTORY.rst for all changes and one of the docs/*-api.rst files for new API

  To build the docs locally:

  pip install -r requirements.readthedocs.txt
  cd docs
  READTHEDOCS=True make html

This is al alternative approach to #185. I decided to make it a separate PR as they are vastly different. Apologies for repo clutter. I am not sure if what I have done here is what you were expecting @sjperkins. I really am just feeling my way at this point. This doesn't subclass pyrap.tables.table but will create more churn. As I mentioned in the other PR, care has to be taken as some functions access tables via _table_future and some access functions via the TableProxy.