Add {switch_handler, Module} return value to cowboy_rest

Also {switch_handler, Module, Opts}.

Allows switching to a different handler type. This is
particularly useful for processing most of the request
with cowboy_rest and then streaming the response body
using cowboy_loop.

doc/src/guide/rest_handlers.asciidoc

@@ -43,8 +43,12 @@ you need.
 All callbacks take two arguments, the Req object and the State,
 and return a three-element tuple of the form `{Value, Req, State}`.
 
-All callbacks can also return `{stop, Req, State}` to stop execution
-of the request.
+Nearly all callbacks can also return `{stop, Req, State}` to
+stop execution of the request, and
+`{{switch_handler, Module}, Req, State}` or
+`{{switch_handler, Module, Opts}, Req, State}` to switch to
+a different handler type. The exceptions are `expires`
+`generate_etag`, `last_modified` and `variances`.
 
 The following table summarizes the callbacks and their default values.
 If the callback isn't defined, then the default value will be used.

doc/src/manual/cowboy_rest.asciidoc

@@ -25,11 +25,15 @@ init(Req, State)
 Callback(Req, State)
     -> {Result, Req, State}
      | {stop, Req, State}
+     | {{switch_handler, Module}, Req, State}
+     | {{switch_handler, Module, Opts}, Req, State}
 
 terminate(Reason, Req, State) -> ok  %% optional
 
 Req    :: cowboy_req:req()
 State  :: any()
+Module :: module()
+Opts   :: any()
 Reason :: normal
         | {crash, error | exit | throw, any()}
 
@@ -51,7 +55,14 @@ implemented. They otherwise all follow the same interface.
 
 The `stop` tuple can be returned to stop REST processing.
 If no response was sent before then, Cowboy will send a
-'204 No Content'.
+'204 No Content'. The `stop` tuple can be returned from
+any callback, excluding `expires`, `generate_etag`,
+`last_modified` and `variances`.
+
+A `switch_handler` tuple can be returned from these same
+callbacks to stop REST processing and switch to a different
+handler type. This is very useful to, for example, to stream
+the response body.
 
 The optional `terminate/3` callback will ultimately be called
 with the reason for the termination of the handler.