Fix problem with multiple .tap on SupplyOperations

Spotted and suggested by retupmoca++ .  Basically, a new internal tap was
being created for each tap on the outside: now only the first outside tap
creates an internal tap.
Let's try to do this another way

This reverts commit 9a1c529013bf580d5c27579559dc9ebe7d71e725.
Remove the experimental "paused" functionality
Add helper method Supply.taps for debugging
Attempt to fix SupplyOperation wrongness (take 2)

Each source_tap now only passes messages on to that one client tap, not
all of them.