Clarify filters interface

[jcchavezs]

Currently there are some gaps in the spec related to filtering:

1. When applying a filter, do we expect the instrumentation to collect any data or not at all? Personally I'd say we expect it to collect request data but no response data.
2. clarify whether ordering is relevant for filters or not
3. Adds support for URL
4. Is headersAndBody still supported?
5. Make it explicit filter for body only works when body capture is enabled

Ping @ryanericson @davexroth @pavolloffay

[ryanericson]

1. I thought it would be ideal too to capture all request data even though request is blocked. But as I understand it the point of the filter interface to evaluate separate part of the request (i.e. EvaluateURL, EvaluateHeaders, EvaluateBody is that we want to call them at different points? Otherwise why not just have EvaluateRequest and call it after all request data capture is done? @davexroth?

2. headersAndBody not needed, don't see it in current implementation for Java too.