Work around casacore getcolslice caching

[sjperkins]

• [x] 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

• [x] 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

[sjperkins]

@JSKenyon Could you please review?

With this PR, I now see the following behaviour https://github.com/ska-sa/dask-ms/issues/92#issuecomment-618380352

This change may have impacts on I/O but I believe its an acceptable change for now and I'd be happy to review other proposed solutions in future.

[sjperkins]

It also might be reasonable to set the cache size to 4MB in the case of getcolslice cases

[sjperkins]

Although https://github.com/casacore/casacore/issues/1018#issuecomment-618333792 would mitigate against a 4MB cache.

[JSKenyon]

Going to plumb this in now. Will let you know how it goes.

[JSKenyon]

I have tried this out - seems to work correctly. There is a small impact on performance but well worth the hit to prevent the memory bloat.