Improve support for message batches

Some tests started failing after changing the message bundler to transfer-queue.

I have added the DONT_BUNDLE flag so that batched messages are delivered on separate threads and the test failures went away (the bundler ignores the flags and bundles messages anyway). But the DONT_BUNDLE flag also reduces performance in some scenarios, so we need to remove it eventually.

First step is to modify tests so that they do not block JGroups threads while waiting for external events (e.g. another message that might be delivered in the same batch).

Second step is to investigate parallelizing the delivery of batched messages in JGroupsTransport. E.g. we could submit a task to the non-blocking pool for the first N-1 messages in a batch and only process the Nth message in the JGroups thread.