MINOR: http: add a new "replace-path" action

MINOR: http: add a new "replace-path" action

This action is very similar to "replace-uri" except that it only acts on the
path component. This is assumed to better match users' expectations when they
used to rely on "replace-uri" in HTTP/1 because mostly origin forms were used
in H1 while mostly absolute URI form is used in H2, and their rules very often
start with a '/', and as such do not match.

It could help users to get this backported to 2.0 and 2.1.