胡斌 / srs_release

add forward_peer,to support forward between origins.the streams pushed from peer
 origins will not be forward any more.
remove forward_server_srs,which used to indicate the forward target server is or is not srs,
add forward_server_other, to list the target rtmp server other than srs in.

support forward between two origin

add config directive:forward_in_turn

update srs2.0release to commit: ff87318b95ec8282ed17e2782724c5fa51f1b61f