Remove the fillcount from the isc_mempool API

Remove the fillcount from the isc_mempool API

Previously, the fillcount could be used to indicate how many elements
would be preallocated every time the memory would be empty.  This would
result in bursty allocations when the mempool would be drained.

For more smooth performance, we allocate the new pool items only as
needed.  In the future, we could consider changing the
isc_mempool_create() function to take an initial number of pre-allocated
items on the pool, so the bursty allocation happens only on the pool
creation.