BUG/MEDIUM: servers: Only set SF_SRV_REUSED if the connection if fully ready.

BUG/MEDIUM: servers: Only set SF_SRV_REUSED if the connection if fully ready.

In connect_server(), if we're reusing a connection, only use SF_SRV_REUSED
if the connection is fully ready. We may be using a multiplexed connection
created by another stream that is not yet ready, and may fail.
If we set SF_SRV_REUSED, process_stream() will then not wait for the timeout
to expire, and retry to connect immediately.

This should be backported to 1.9 and 2.0.
This commit depends on 55234e33708c5a584fb9efea81d71ac47235d518.