Warm up PowerShell Worker before receiving function load request

[AnatoliB]

Resolves #485

• The first PowerShell instance creation is moved to Worker.Main, and it is now performed immediately after starting the process, not waiting for the function load request. It is important to remember that at this point we cannot initialize $env:PSModulePath because the app root path is not resolved yet. So, for the first PowerShell instance, this step is deferred until the function load request and performed in RequestProcessor.SetupAppRootPathAndModulePath. For all the subsequent PowerShell instances, $env:PSModulePath comes from the initial session state, as before.
• Handling the function load request involves creating a PowerShell function from a script file (see PowerShellManager.DeployAzFunctionToRunspace), and this takes hundreds of milliseconds even for a short script when performed for the first time. However, the subsequent invocations are significantly faster. So, creating and removing a dummy function proactively saves noticeable time on the function load request (see Worker.WarmUpPowerShell).

This reduces cold start time by about 4 seconds (measured on App Service plan, S1 instance):

Stage	Before the change (ms)	After the change (ms)
Function load request	3700	200
First function invocation request	2200	1700
Total	5900	1900

[Francisco-Gamino]

Thanks @AnatoliB for sending this out!

A couple of minor comments. Other than that, LGTM.