Implement the Alternative concurrency mechanism on an exemplar for MCCS

This feature has been done. When the feature was defined, ska-tango-base only offered a single concurrency mechanism: the task executor. It was recognised that this was not fit for all use cases. For example, monitoring and control of hardware would normally be achieved through use of a poller, and if you have a poller, you already have an alternative concurrency mechanism. Accordingly, a generic polling mechanism was developed. This was applied to a range of repositories, including ska-ser-test-equipment (the ITF Test Equipment M&C repository, ska-low-cbf-proc (the repo for M&C of CBF Alveo cards) and ska-low-mccs-spshw (in the Subrack Tango debice). Once fully mature, the generic polling mechanism was upstreamed to ska-tango-base, and the aforementioned repos were refactored to use it. Thus the current situation is that ska-tango-base now has an alternative concurrency mechanism in the form of a generic polling mechanism, which is already in use in multiple downstream repositories.