Add isc_mem_reget() function to realloc isc_mem_get allocations

The isc_mem_get() and isc_mem_put() functions are leaving the memory allocation size tracking to the users of the API, while isc_mem_allocate() and isc_mem_free() would track the sizes internally. This allowed to have isc_mem_rellocate() to manipulate the memory allocations by the later set, but not the former set of the functions.

This commit introduces isc_mem_reget(ctx, old_ptr, old_size, new_size) function that operates on the memory allocations with external size tracking completing the API.