Elevate common request headers to attributes

[geeknoid]

We need to move common request headers to be first class attributes in order to improve cache efficiency. If the Mixer references any request headers during a Check call, the cache entry ends up being invalidated whenever any request header changes, which is the normal case. This effect completely neutralizes the existing cache.

[qiwzhang]

This will cause data duplication. The right solution is to support string map subkey in ReferencedAttributes.

[geeknoid]

This can be the long term solution when it is paired with issue #178

[mandarjog]

related to Mixer issue of istio/old_mixer_repo#1099

[geeknoid]

Since we now handle individual request headers correctly, no need for this.