Advanced RoundRobin policy
RoundRobin should have the flexibility to allow one or more senders to "drop out", allowing runs to continue in case of EventBuilder crash/hang.
Given the extremes of possible RoundRobin configurations:
"Strict" RoundRobin: All senders must be present, store all remaining tokens for later
"Loose" RoundRobin: Use all tokens, regardless of how many senders remain
Allow users to configure Strict mode, Loose mode, or modes gradually looser from Strict mode, or modes gradually stricter from Loose mode.