Performance tests for the invocation loop

[fabianfett]

re-enable the performance test that was available in v1.

This requires

• MockServer to be Swift 6 compliant
• re-implement the MAX_REQUEST that was available in v1 (to avoid an infinite loop when pointing the runtime against the mock server)

[sebsto]

Other ideas

• Implement a test for https://maxday.github.io/lambda-perf/
  and contact the owner to add the runtime 2 to the list of bench

• local performance with swift-benchmark package