add with_connection_proxy method

`ActiveRecord::Base` now has the `with_connection` method, which is like
the old `connection` method on `ActiveRecord::Base` but it yields the
connection to a block instead of returning it:
rails/rails@22f41a1

This new method is now used for transactions, so in order for
transactions to work we need to implement an equivalent in Replica
Pools.

lib/replica_pools/active_record_extensions.rb

@@ -17,6 +17,10 @@ def connection_proxy
         ReplicaPools.proxy
       end
 
+      def with_connection_proxy
+        yield ReplicaPools.proxy
+      end
+
       # Make sure transactions run on leader
       # Even if they're initiated from ActiveRecord::Base
       # (which doesn't have our hijack).

lib/replica_pools/hijack.rb

@@ -18,6 +18,7 @@ def inherited(child)
     def hijack_connection
       class << self
         alias_method :connection, :connection_proxy
+        alias_method :with_connection, :with_connection_proxy
       end
     end
   end