add buffer state of disabled diagnostics state

[prabirshrestha]

This is part of Diagnostics Revamp.

Helps for issues like these related to easymotion. #915

Currently this make sure the internal diagnostics state is up to date. It is still up to the ui implementations to respect the flag since get_all_diagnostics* function doesn't respect it for perf reasons. Updates to ui respecting this flag will come in separate PRs.

When I start updating UI to respect this I will expose a public documented api which will look something like this.

autocmd User EasyMotionPromptBegin call lsp#disable_diagnostics_for_buffer(bufnr('%'))<CR>
autocmd User EasyMotionPromptEnd call lsp#enable_diagnostics_for_buffer(bufnr('%'))<CR>

While I could have a method to disable per diagnsotics ui it made sense to do this for entire diagnostics features.