Write proposal for n23 patch behavior

In legacy/trac#4488 (moved) we located and updated the n23 patch from the Defenestrator paper. It's now in branch 'n23-2' in my git repo.

The next step is to refactor connection_or_consider_sending_flowcontrol_cell(). To do that, we need to know what the function is supposed to be doing.

Under what circumstances do we send a flowcontrol cell, and how do we pick the values for it? Just as important, in what cases (e.g. if we're a client) do we not send flowcontrol cells?

I'm hoping Mashael will do this, with Ian's help.