(WIP) Add byte-based flood option

[thommey]

Patch by: remorse One-line summary: Add byte-based flood option Fixes: #242

This is to track the floodsize branch. We might want to expand our "couplets" to be X:Y:A:B for X lines in Y seconds or A bytes in B lines (or similar), or make the size trigger its own flud bind (current patch calls pub). It is confusing for users to set flood-chan to 0:0 and still triggering a flood. Also, size limit isn't displayed in chaninfo AFAIK.