There is a server-side limit on the maximum size of ACK and modACK
requests, which can be hit if the leaser tries to manage too many
messages in a single requests.

This commit assures that such large requests are split into multiple
smaller requests.

The previous limit of 3000 seems to be too optimistic, and the request
size limit is still hit. Reducing the batch size to 2500 fixes the
problem.

The tests should also check that each message is (MOD)ACK-ed exactly
once.