Consolidate HTTP probes

[MikeGoldsmith]

Currently we have two HTTP based probes (net/http and gorilla) and a third to be added in #100. The probes are almost identical apart from the registration name and could be unified. The instrumentors would then deploy the unified probe instead of a custom version. Each probe is based on the net/http ServeHTTP(w ResponseWriter, r *Request) interface.

This would make probe maintenance easier by reducing code and would allow improvements to be applied generically instead. eg HTTP response parsing.

TODOs

• Ensure consts are used for max loop iterations - suggestion from @pdelewski in [feat] Adding context propagation to net/http server instrumentation #92 comment
• Ensure we use constants where possible for indexing - suggestion from @pdelewski comment