Performance testing for open image handling

[donaldgray]

Verify that there has been no performance degradation or bottlenecks serving open tiles vs previous implementation.

[donaldgray]

Ran tests locally and in deployed dev version. Noticed following

• Protagonist orchestrator is slightly faster than deliverator for making first requests for tiles (ie uncached).
• Bottleneck in performance comes from using single iip image instance. Protagonist doesn't appear to slow down these requests much
• Deliverator is considerably faster for subsequent tile requests as tiles are served from varnish.

We will look to add varnish in future tickets, alternatively use (or write) a diskbased response cache implementation. See dotnet/yarp#120