HPCC-12633 Improve contention cost in splitter

In cases where the consumers keep pace and are waiting on the
produce side of the splitter, the cost of waiting was costly.
The change here, introduces a callback mechanism into the shared
write ahead, such that the producer [optionally] only triggers
the readers that there is something to consumer per page flush.
In the memory variety of the shared writer which blocks when it
is far enough ahead, the producer will write (and page) as fast
as possible on a async thread, and block when full.
All of this savs the cosumers, entering crits, waiting on
semaphores and waking up and re-entering crits etc. on a per row
basis when they are consuming quickly.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
It's probably not what I intended..

A thread _could_ update pendingWrite to false, as another just passes the check for pendingWrite==true in the crit block at the start.. As a result this other thread might miss a writeahead it had really wanted and block. However, the 1st thread (that updated pendingWrite to false) would still end up calling writeahead successfully.

So I don't think it can cause deadlock, it isn't idea in that rare scenario.

I should protect the update to false with a separate crit block.