Break SymbolReferenceFinder into its own class.  This makes the merge with another branch much cleaner.

Do not keep around references to Project instances.  We can just use the semantic version of the project to know if we need to reprocess it.

Produce SymbolTreeInfo indices in the background.  only search them if they're available.

made metadatareference check in compilation tracker to actually work.

simplify some implementation and add comments and etc

Tweak ordering of NavigateTo items.

Simplify test workspace construction

we only need to care about CompilationReference since other metadata reference we explicitly make sure we use same reference unless it is changed.

added unit test for reference equality

Add CodeFix to convert async methods to non-async.

Add shutdown argument to VBCSCompiler

Now, Add-Using will use the indices if they have been built.  Or not search
those projects/metadata if they're not available.

Note: this doesn't change any of the searching of the current project.  That
works the same way it always has.  This is only for searching unreleated projects
and metadata dlls.

Otherwise, if we use 'Compilation.References' instead, then the compiler will spend a ton
of time scanning through those references looking for #r directives.  The compilation tracker
does not care about these, and is only checking actual declared project references.