AMDS: refine feature for enable/disable buffergroup #118
Description
When working on AMDS PV for SGM, I "think" that sometime the user might want to have a full list of configured PVs and disable some of them. In this case, when the PV (bufferGroup) is disabled, we should be disable bufferGroup update as well. That is, we don't need to add new data to that bufferGroup and the bufferGroup won't handle dataRequest any more. (an error message should return to the client).
The bufferGroup should also be able to handle configuration request, so that we can enable/disable a bufferGroup on demand.