HPCC-9489 Add support to roxie for a checking memory allocator

Can be enabled per-query using @checkingHeap, or globally via
control:checkingHeap or adding checkingHeap=true in the config file.

Note - this is deliberately NOT exposed in configmgr - it's not something
people should enable unless specially instructed.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>