Add a static pre-allocated buffer to isc_buffer_t

Add a static pre-allocated buffer to isc_buffer_t

When the buffer is allocated via isc_buffer_allocate() and the size is
smaller or equal ISC_BUFFER_STATIC_SIZE (currently 512 bytes), the
buffer will be allocated as a flexible array member in the buffer
structure itself instead of allocating it on the heap.  This should help
when the buffer is used on the hot-path with small allocations.