CAChannel - thread contexts

we currently do a bit of magic trying to re-use thread contexts, probably better to subclass python thread and use this to create and destroy a context based on its lifetime. See https://pyepics.github.io/pyepics/ca.html#epics.ca.CAThread for an example, though this connects to initial context and we'd need option to create own context too. More more info on ca contexts see https://pyepics.github.io/pyepics/advanced.html#advanced-threads-label

we should also make it easier to choose the thread per context v single shared context in genie_python rather than setting an environment variable to "pretend" it is a pcaspy server like blockserver or reflectometry server. See https://github.com/ISISComputingGroup/InstrumentChecker/pull/64