Rename 'halt' to 'stop' for better consistency

Now everywhere in Cowboy when we want to stop something we return
a 'stop' tuple instead of one of the many choices depending on
context that we had before.

This particular change affects middlewares, sub protocols and
REST handlers which were using 'halt' to stop processing.

doc/src/manual/cowboy_middleware.ezdoc

@@ -21,7 +21,7 @@ optionally with its contents modified.
 : execute(Req, Env)
 	-> {ok, Req, Env}
 	| {suspend, Module, Function, Args}
-	| {halt, Req}
+	| {stop, Req}
 
 Types:
 
@@ -41,7 +41,7 @@ The `suspend` return value will hibernate the process until
 an Erlang message is received. Note that when resuming, any
 previous stacktrace information will be gone.
 
-The `halt` return value stops Cowboy from doing any further
+The `stop` return value stops Cowboy from doing any further
 processing of the request, even if there are middlewares
 that haven't been executed yet. The connection may be left
 open to receive more requests from the client.

doc/src/manual/cowboy_rest.ezdoc

@@ -58,7 +58,7 @@ stacktrace of the process when the crash occurred.
 
 :: Callbacks
 
-: Callback(Req, State) -> {Value, Req, State} | {halt, Req, State}
+: Callback(Req, State) -> {Value, Req, State} | {stop, Req, State}
 
 Types:
 
@@ -72,7 +72,7 @@ on the `Value` type, the default value if the callback is
 not defined, and more general information on when the
 callback is called and what its intended use is.
 
-The `halt` tuple can be returned to stop REST processing.
+The `stop` tuple can be returned to stop REST processing.
 It is up to the resource code to send a reply before that,
 otherwise a `204 No Content` will be sent.
 

doc/src/manual/cowboy_sub_protocol.ezdoc

@@ -8,8 +8,7 @@ by modules that implement a protocol on top of HTTP.
 : upgrade(Req, Env, Handler, Opts)
 	-> {ok, Req, Env}
 	| {suspend, Module, Function, Args}
-	| {halt, Req}
-	| {error, StatusCode, Req}
+	| {stop, Req}
 
 Types: