HPCC-8297 Provide a callback mechanism when resizing a row

When using resizeRow to resize a buffer which is registered with the memory
manager, there needs to be a way of atomically updating the memory buffer
variables.  Otherwise you can get inconsistent results or deadlock the
system.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>