Investigate Toolkit performance after explorer plugins are registered

This issue was created from looking at the following toolkit log (v1.17.1.0)

> @awschristou I have attached the log file. I hope it will help.
[log.txt](https://github.com/aws/aws-toolkit-visual-studio/files/4762830/log.txt)

_Originally posted by @M-Imtiaz in https://github.com/aws/aws-toolkit-visual-studio/issues/14#issuecomment-642423305_

There was a noteworthy 11-second gap during one of the toolkit activations:

```
2020-06-10 17:04:27,244 [26] DEBUG Amazon.AWSToolkit.ToolkitFactory - Finished Registering Toolkit Plugins in the AWS Explorer...

2020-06-10 17:04:38,259 [1] DEBUG Amazon.AWSToolkit.VisualStudio.Lambda.LambdaTesterEventListener - Initializing Lambda Tester event listener
```

I suspect this is related to account validation. We should add more logging to help get a better sense of performance bottlenecks.

Start with the toolkit log in log.txt and the ToolkitFactory and LambdaTesterEventListener entries around plugin registration and listener initialization. Investigate the 11-second gap, including the suspected account-validation step, and add logging that identifies the performance bottleneck.

Written by the indexing model from the issue text.