MINOR: http-rules: Make replace-header and replace-value custom actions

MINOR: http-rules: Make replace-header and replace-value custom actions

Now, these actions use their own dedicated function and are no longer handled
"in place" during the HTTP rules evaluation. Thus the action names
ACT_HTTP_REPLACE_HDR and ACT_HTTP_REPLACE_VAL are removed. The action type is
now set to 0 to evaluate the whole header or to 1 to evaluate every
comma-delimited values.

The function http_transform_header_str() is renamed to http_replace_hdrs() to be
more explicit and the function http_transform_header() is removed. In fact, this
last one is now more or less the new action function.

The lua code has been updated accordingly to use http_replace_hdrs().