Improve poll_transmit loop logic

[flub]

poll_transmit has a loop in it which iterates over the spaces. In multipath this means it iterates over the spaces & paths since we haven't managed to merge that, even though logically it is all a space.

At a high level it checks two things:

• Do we have CIDs to send on the space?
• Do we have anything to send on the space?

Currently the logic to find the next is repeated and not very easy to follow.

But there's another issue, it would be safer if we could also check for a 3rd thing:

• Do we have crypto keys for the space?