Changes to make the inter-packet pause of the filter executable changeable.

__Merges into `serial`__

@mossblaser If you're happy then please merge this into the serial branch.

Adds a new word to the system region of the FilterVertex.  Value of this word is used to control the period of time between transmitting packets.

No warning made to the user as this is quite low-level, we should probably try to do many more graceful things wrt. IO but currently don't.