Crash when intellisense is requested before PSIC is finished initializing

[SeeminglyScience]

Repro steps

1. Have the 2.0.0 preview build
2. Put Start-Sleep 5 in your $profile (or just have a really poorly optimized profile like myself)
3. Restart the PSIC and type in the editor

PowerShellContext should delay InvokeOnPipelineThread requests until initialization is completed.

[rjmholt]

Ok I've been trying to get a proper debug repro of this, but getting the debugger attached prevents the race condition that causes the problem.

@SeeminglyScience I feel like you've got a pretty good idea for what's causing this, so I might let you handle it if that's alright.

[rjmholt]

I should mention that I did see it coming from ForcePSEventHandling() in one case.