Adds three new template placeholders that resolve into request IP address similar to existing Source, SourceFromLast and ClientIP predicates.

• could be handy to support default key in "Configurable consistent hash request key" (Configurable consistent hash request key #1711)
• allows client rate limit based on last value of X-Forwarded-For (XForwardedForLookuper supports only first value)
• named after existing predicates to avoid confusion (could be extended if/when "Configurable ClientIP utility function" (Configurable ClientIP utility function #1620) is implemented)

Signed-off-by: Alexander Yastrebov [email protected]