add document of message passing version2

this version is simpler than before
add *.swp to .gitignore
fix a typo: md_send_metadat_update to md_send_metadata_update
change dlm_md_idle to dlm_md_token to keep align with the documentation
rewrite message transmitting protocol

raid1_sendd: a thread for sending message
raid1_recvd: a thread for receiving message
replace struct msg_suspend,msg_metadata_update,msg_resync_finish with struct cluster_msg for message passing
fix typos

1, sequence number in doc/message-update.txt
2, mddev->avail_bitmap[1] in raid1.c