BUG/MINOR: proto_reverse_connect: support SNI on active connect

BUG/MINOR: proto_reverse_connect: support SNI on active connect

SNI may be specify on a server line for connecting to the remote host.
This requires to manually set it on the connection via
ssl_sock_set_servername().

This step was missing when a server line was used for active reverse
HTTP. Fix this by adding the missing ssl_sock_set_servername()
invocation inside new_reverse_conn().

Note that for the moment, no session is instantiated to carry active
reverse connection. A direct consequence of this is that SNI sample
retrieval may crash depending if it depends on session parameters. This
should be fixed by a later commit. In the meantime, this patch is
sufficient to support simple SNI value such as constant expressions.

No need to backport.