Use static storage for isc_mem water_t
On the isc_mem water change the old water_t structure could be used after free. Instead of introducing reference counting on the hot-path we are going to introduce additional constraints on the isc_mem_setwater. Once it's set for the first time, the additional calls have to be made with the same water and water_arg arguments.
Closes #2831 (closed)
Edited by Ondřej Surý