steam-http: fixed segfaults occurring with reattempted requests

All requests must remain in the request queue until they are have been
completely handled, this includes requests that are being reattempted.
This ensures reattempted requests are closed with all other requests in
the SteamHttp, otherwise segfaults and memory leaks occur.
steam: fixed the handling of certain relationship messages

Currently, relationship messages are always handled, even if the
friendship state has not actually changed. This behavior can be seen
occasionally when connecting, and all Steam friends are "added" even
though they are an established Steam friend.