Switched to using posted next events after sendfile_max_chunk.

Previously, 1 millisecond delay was used instead.  In certain edge cases
this might result in noticeable performance degradation though, notably on
Linux with typical CONFIG_HZ=250 (so 1ms delay becomes 4ms),
sendfile_max_chunk 2m, and link speed above 2.5 Gbps.

Using posted next events removes the artificial delay and makes processing
fast in all cases.