flowcontrol.fixedLimiter: Swap in semaphore.Weighted.

[zenhack]

...sadly, this does not fix my deadlock, but even so, no need to re-implement this ourselves.

[lthibault]

I haven't seen this particular error message yet, but may share the same underlying cause as the race condition triggered by #232 ?

I'm okay to merge this if you're confident this isn't a new bug.

=== RUN   TestRecvDisembargo
Error:     level0_test.go:1982: conn error: <nil>
Error:     level0_test.go:1982: conn error: send return: %!w(<nil>)
Error:     level0_test.go:1982: conn error: <nil>
Error:     level0_test.go:1982: conn error: <nil>
Error:     level0_test.go:1982: conn error: send return: %!w(<nil>)
Error:     level0_test.go:1982: conn error: <nil>
Error:     level0_test.go:1982: conn error: send return: %!w(<nil>)
Error:     level0_test.go:1982: conn error: send return: %!w(<nil>)
Error:     level0_test.go:1982: conn error: rpc: incoming disembargo: answer ID 55 has not sent return
Error:     level1_test.go:761: don't know how to handle abort; skipping
Error:     level1_test.go:668: recvMessage(ctx, p2): rpc pipe: receive on closed pipe
--- FAIL: TestRecvDisembargo (0.01s)

[zenhack]

That test doesn't use fixedLimiter, so I'm pretty confident. Merging.