Implemented support for request groups with holds in core code, Demo dri...

...ver and Voyager drivers.

Note that this part 1 of at least 2 parts, and contains only partial implementation for "checkItemsExist" and "checkItemsNotAvailable" where it's intertwined with the request group functionality and no implementation for "disableAvailabilityCheckForRequestGroups". The final implementation for the rest will be part 2, but I'm trying to keep this as simple as possible. If you wish for me to add everything in a single branch, I'm fine with that too.