Make credit flow state transition interval 1s instead of 5s

Management UI and HTTP API currently report connections and channels
as in flow if they've been in flow for the last 5 seconds. That
can confuse the user with inter-node flow control, making them
believe the flow is permanent (it is not: the actual state
toggles many times a second).

This reduces the interval to 1s, which seems more reasonable and
accurate (in a way).

Fixes #138.

src/credit_flow.erl

@@ -68,6 +68,11 @@
             put(Key, Expr)
         end).
 
+%% If current process was blocked by credit flow in the last
+%% STATE_CHANGE_INTERVAL milliseconds, state/0 will report it as "in
+%% flow".
+-define(STATE_CHANGE_INTERVAL, 1000000).
+
 %%----------------------------------------------------------------------------
 
 %% There are two "flows" here; of messages and of credit, going in
@@ -117,7 +122,7 @@ state() -> case blocked() of
                false -> case get(credit_blocked_at) of
                             undefined -> running;
                             B         -> Diff = timer:now_diff(erlang:now(), B),
-                                         case Diff < 5000000 of
+                                         case Diff < ?STATE_CHANGE_INTERVAL of
                                              true  -> flow;
                                              false -> running
                                          end