Tighten type bounds for assertInstanceOf

This makes it possible to catch checks for completely unrelated types
at compile time. E.g.,

    assertInstanceOf("foo", Integer.class)

is a compile-time error.
Fix thread count for Jetty 9

Jetty 9 requires more threads than Jetty 8.
Collapse and deprecate AsyncHttpClient
Test dynamic BodyGenerator
Test JettyHttpClient.executeAsync()
Fix excessive log messages for HTTP tests
Configure Jetty to use java.util.Logging
Make http-client a test dependency for http-server
Update to airbase 16