MINOR: h2: retrieve the front proxy from the caller instead of the session

MINOR: h2: retrieve the front proxy from the caller instead of the session

Till now in order to figure the timeouts, we used to retrieve the proxy
from the session's owner, but the new API provides it so it's better to
simply take it from the caller at init time. We take this opportunity to
store the pointer to the proxy into the h2 connection so that we can
reuse it later when needed.