Performance: Stop passing entire buffer to OmniSharp on every keystroke

[DustinCampbell]

It's not clear why, but the entire buffer gets sent to OmniSharp on every keystroke. This results in poor typing performance in all but the smallest files. We should be passing just the changes.

https://github.com/OmniSharp/omnisharp-vscode/blob/master/src/features/changeForwarding.ts#L25

[rchande]

@DustinCampbell Do you think we should still do this?

[DustinCampbell]

Absolutely. I even mentioned this on the last OmniSharp phone call.