BBR FlowLimiter underestimates bandwidth

[zenhack]

As a follow up to #294, while the flow limiter partially works, there are bugs, and in particular it seems to often under-estimate the correct bandwidth, capping it at basically one message on the wire at a time. Due to the sheer size of the PR and the fact that no existing functionality was broken, we decided to merge it without fixing this -- but we should actually fix it.