(Cleanup proposal.)

Currently, access log filters are manually evaluated in each implementation. Filters are instantiated and passed to the AccessLog::Instance, and then each AccessLog::Instance's log() implementation has logic of if (filter_ && filter_.evaluate(...)) then log(). Instead, I propose we move that logic up to some sort of public bool evaluate_filter(...) function in the AccessLog interface, and wrap all of Envoy's AccessLog::Instance::log() calls in a conditional.

cc: @ambuc @eziskind @AndresGuedez