The admin handler and tests are getting out of control. We should decomp these files into multiple independent files for different handlers, the filter, etc.

https://github.com/envoyproxy/envoy/blob/master/source/server/http/admin.h
https://github.com/envoyproxy/envoy/blob/master/source/server/http/admin.cc