AMSI scan should be done outside the static lock #3919

Currently our AMSI code (https://github.com/PowerShell/PowerShell/blob/master/src/System.Management.Automation/security/SecuritySupport.cs#L1672) calls the AmsiScanString() method within a static lock.

This is unnecessary because the AMSI API is thread safe and designed to process multiple scan requests.

But this code also needs to be refactored because it is unnecessarily un-initializing and re-initializing AMSI when multiple runspaces are used. In addition it is not tracking pipeline sessions correctly for multiple runspaces.

So I see the work items as:

1. Ensure AMSI is initialized and un-initialized only once (see related issue Transcription for the host stops, when ANY runspace in the host get closed #2334).
2. Add concurrent dictionary to correctly track AMSI session per pipeline.
3. Don't call AmsiScanString within the static lock.