Upgrade to airbase 17
Add back support for legacy http-client.threads config
Make field final

Thread-safety/visibility issue since the class can be called from arbitrary threads in Jetty.
Add back http-client.max-content-length configuration option

Max content length option was accidently removed during migration to new
JettyHttpClient.  The default value is still 16MB.
Add 0.86 release notes
[maven-release-plugin] prepare release 0.86
[maven-release-plugin] prepare for next development iteration