HPCC-12409 Pathological distribute slowdown fix

Prevent sender queing partial buckets if writers already actively
working on buckets to same target.
Also allow multiple write handler threads to work on the same
target, this allows the serialization/compression (and potentially
dedupping) to occur in parallel even if to same writer.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
I'm not sure it's _really_ worth it.. but you're right not likely to change much and handled by candidates loop below.