number of ul slots: auto

[Harold]

Easy idea (not new) - just slowly add extra upload slots if the upload speed cap is not met, slowly remove them if it is met - should balance out just about right

Helps to give peers proper speeds without their needing a gazillion download connections

May also help to counter the urge some people have to set the number of upload slots far too high

edit: note that this is not the same as "don't count slow connections" at all

it's also not the same as "use additional upload slots if speed < 90%" - this should go in the other direction as well (eg "use less upload slots if speed >99%")

edit: perhaps the number of queued but not yet sent blocks might be a better heuristic than the total upload speed?