Improve the speed of batch imports

Currently, Mailman Core will try to regenerate the LMTP files on every list create/delete event. This might okay when a new list gets created, but it might not be desirable when someone is trying to create hundreds or thousands of lists at the same time as it would keep re-generating the LMTP files from scratch.

There should be a flag/environment variable to signal Mailman to skip creation of these LMTP files.