Only publish diagnostics when they change

[radeksimko]

Background

Previously we would re-publish any (if any) diagnostics as part of RPC request handlers textDocument/didOpen, textDocument/didChange etc. Since #714 and also because we always maintain a cache of diagnostics in memdb we have an opportunity to compare new vs old diagnostics and only publish diagnostics if they actually changed and avoid unnecessary RPC chatter.

This is what gopls reportedly does: microsoft/language-server-protocol#1217

Use-cases

Reduce the amount of RPC notifications sent for publishing diagnostics from the server to the client.

Attempted Solutions

Re-publish any time any aspect of the module changes (#714).

Proposal

• Implement equality comparison for diagnostics
• TODO: figure out how and when do validate diagnostics get cleared