Add support for bulkcreates
After discussing with @colemanw on chat - there is a demand in some cases to create an entity in a way that performs well on multiple creates at one. GroupContact or MailingQueue are both examples. The current example is creating bulk custom fields - the actual row saves are fine here but if adding more than one field to an-already-large custom table then multiple column adds is slow whereas one sql action adding multiple indexes & columns (& one for log tables) is much better.
We talked about laying the ground work for this being a supported apiv4 action with the goal being that apiv4 would expose an action for any entities that have a bulkCreate action.
At this stage my scope is limited to cleaning up the CustomField.create function and adding a bulkCreate function that is tested & suitable to be exposed via apiv4 (but I'm not taking that next step at this stage so the contract can still change). Currently 2 BAO have bulkCreate functions. They relate to mailings and are suitable for bulk create although require some tweaks (passing keyed params rather than 0, 1 etc)