Ensure that connecting to Rabbit is threadsafe

When there are competing publisher threads, there is a chance that the
publisher has not been initialized when Hutch is registered as
connected. The following diagram should shed some light on the race
condition:

```
        Thread 1                  Hutch                   Thread 2
        --------                  -------                 ---------
t1      Hutch.publish()  --->  connected? (FALSE)
                           |-> open_connection!
t2                             connected? (TRUE)  <--- Hutch.publish()
                               @broker.publish()   |-> NoMethodError (@publisher)
t3                         |-> declare_publisher!
        PUBLISHED        <-|   @broker.publish()
```

Wrapping the `Hutch.connect` call should resolve this issue.

lib/hutch.rb

@@ -14,6 +14,8 @@
 require 'hutch/tracers'
 
 module Hutch
+  @@mutex = Mutex.new
+
   def self.register_consumer(consumer)
     self.consumers << consumer
   end
@@ -40,10 +42,12 @@ def self.global_properties
   # @param config [Hash] Configuration
   # @option options [Boolean] :enable_http_api_use
   def self.connect(options = {}, config = Hutch::Config)
-    return if connected?
-
-    @broker = Hutch::Broker.new(config)
-    @broker.connect(options)
+    @@mutex.synchronize do
+      unless connected?
+        @broker = Hutch::Broker.new(config)
+        @broker.connect(options)
+      end
+    end
   end
 
   def self.disconnect