Draft: Use isc_mem_reget() when growing buffer dynamically

Previously, we cannot use isc_mem_reallocate() for growing the buffer dynamically, because the memory was allocated using the isc_mem_get()/isc_mem_put() API. With the introduction of the isc_mem_reget() function, we can use grow/shrink the memory directly without always moving the memory around as the allocator might have reserved some extra space after the initial allocation.