Change setmaxcachesize to require a read lock, not a write lock

[sjperkins]

Closes #280

Previously, setmaxcachesize was called with a write lock, but this was problematic for readonly tables as setmaxcachesize was called during read ops.

As setmaxcachesize only modifies the internal casacore column caching, this can safely be made a read operation.

• [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]

@landmanbester Do you want to give this branch a try?

[landmanbester]

Thanks @sjperkins. I've kicked it off, so far so good

[landmanbester]

It ran through successfully so I think we are all good. Thanks