HPCC-11968 Don't use unique allocators per bucket

Unique allocators per bucket, were there as an optimization, to
help pages become free when a bucket spill event occurred.
However, the upshot of having a unique allocator per bucket, was
a reserved roxiemem page per bucket, which meant a very high
initial overhead for the bucket set and premature spilling.

Revert to shared allocator.

Also, avoid rehash() causing other buckets to spill, use
maxSpillCost on table resize to ensure only lower priority
spilling occurs.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>