Use static storage for isc_mem water_t

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.