Allow dispatcher to batch multiple isolated dispatches per destination

[danielmarbach]

Multiple isolated dispatches (possible when the outbox is at play with a handler or a transactional session that sends multiple messages) can be batched for efficiency reasons as explained in

// Changing the dispatch consistency to be isolated to make sure the transport doesn't
// enlist the operations in the receive transaction. The transport might still want to batch
// operations for efficiency reasons but should never enlist in the incoming transport transaction.
// Otherwise a failure to ACK the incoming message after Outbox storage has been set to Dispatched
// would result in outgoing message loss.

https://github.com/Particular/NServiceBus/blob/master/src/NServiceBus.Core/Reliability/Outbox/ForceBatchDispatchToBeIsolatedBehavior.cs#L14-L19

For efficiency reasons it would be possible to change the dispatcher to account for those scenarios instead of doing individual broker calls as long as those operations are not enlisted in the receive transaction.