MINOR: backend: don't always takeover from the same threads

MINOR: backend: don't always takeover from the same threads

The next thread walking algorithm in commit 566df309c ("MEDIUM:
connections: Attempt to get idle connections from other threads.")
proved to be sufficient for most cases, but it still has some rough
edges when threads are unevenly loaded. If one thread wakes up with
10 streams to process in a burst, it will mainly take over connections
from the next one until it doesn't have anymore.

This patch implements a rotating index that is stored into the server
list and that any thread taking over a connection is responsible for
updating. This way it starts mostly random and avoids always picking
from the same place. This results in a smoother distribution overall
and a slightly lower takeover rate.