This change replaces synchronous lock statements in the core analyzer executor and analyzer state tracker (for IDE live analysis) with semaphore slim DisposableWaitAsync invocations. This ensures that most of the analyzer driver and analyzer executor code is now async, with only the eventual callback into the analyzers being a synchronous method ([ExecuteAndCatchIfThrows_NoLock](http://source.roslyn.io/Microsoft.CodeAnalysis/R/02d7df8203d3591e.html))

Testing: I verified that the self-build time of Roslyn.sln (which uses a few analyzers), is almost identical after this change. We are hoping that this will improve build performance when there is lot of lock contention, which has shown up from perf traces (see https://github.com/dotnet/roslyn/issues/6053).

Fixes #6399