Add support for hashcode in open text document request.

[tinaschrepfer]

It would be great if the protocol supports another way of submitting open text document requests where only a file path and hashcode is sent. The server can compute the hash on its side when it has opened the file, and send a response back indicating if the hashes are the same. If the hash is different, the client can send over the entire file content. This would be a performance improvement in that the client only sends over the whole content if it's necessary. For big files, this would make a difference.

[dbaeumer]

To optimize sending the content we could make an opt in into memory mapped files to send the content of large files.