ProjectSystemProject `_gate` gets blocked on client RPC requests causing slow sln load

[dibarbet]

To reproduce

1. Clone https://github.com/MudBlazor/MudBlazor
2. Open in VSCode with C# (2.79.8) + CDK installed
3. Note the loading on most hover, no intellisense, and extremely long solution load (15+ minutes)

I took a trace of the lsp server, it looks like the majority of the thread time (74%) is spent blocked on the _gate semaphore in ProjectSystemProject

waiting to process dynamic file info changed events. It appears as though this gate is slow because under the same _gate, it makes an RPC call to the client to get the latest file info -

roslyn/src/Workspaces/Core/Portable/Workspace/ProjectSystem/ProjectSystemProject.BatchingDocumentCollection.cs

Line 478 in e2b26ff

var fileInfo = fileInfoProvider.GetDynamicFileInfoAsync(

Talked with @jasonmalinowski offline, and think we can move this out of the gate.