Revert "ISPN-4782 Drop the Mux Channel block in favour of a FORK protocol in the"

This reverts commit c150200b20018bbe9c2862764582b110abed6f59.
ISPN-4864 Drop the JGroups FORK protocol until we can properly support it
ISPN-4776 - The topology id for the merged cache topology is not always bigger than all the partition topology ids
ISPN-3938 AdvancedAsyncCacheLoader.process() concurrency issues

Simplify ExecutorAllCompletionService to track the tasks itself instead of
using an ExecutorCompletionService.