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>
Would it be possible/better to use atomic_inc on activeWriters and avoid the critical section?