HPCC-9252 Improve the locking contention in roxiemem

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-9252 Avoid freeing empty pages in a heap which contains none.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-9252 Reduce the locking window when allocating rows from a heap

Also rationalise the names of the classes so the Heap and Heaplet classes
have similar names.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>