Mapping severity types is very slow. Rewrite it to use a dictionary.

[janniksam]

The foreach loop @
https://github.com/alanwales/vsts-resharper-code-quality-task/blob/master/src/task/RunResharperCodeAnalysisTool.ps1#L134
is very slow with a huge amount of found issues. (takes like 3-5 min for 30000 issues on our machines).

A rewrite using a dictionary to map severity types to a severity level would be alot more efficient. The Linq-expression does cost alot of performance.

[tomkerkhove]

Happy to review a PR if you have the bandwidth!