Allow :pool_class option to take a :symbol

This allows you to load connection_pool classes via
:pool_class=>:foo, as long as inside the
sequel/connection_pool/foo.rb file, you set
CONNECTION_POOL_MAP[:foo] = self.

No connection pool class currently uses this, but it's possible
nondefault connection pool classes will be added in the future,
and if so, they will need this.

lib/sequel/connection_pool.rb

@@ -53,7 +53,7 @@ def get_pool(opts = {}, &block)
 
     # Return a connection pool class based on the given options.
     def connection_pool_class(opts)
-      opts[:pool_class] || CONNECTION_POOL_MAP[[!!opts[:single_threaded], !!opts[:servers]]]
+      CONNECTION_POOL_MAP[opts[:pool_class]] || opts[:pool_class] || CONNECTION_POOL_MAP[[!!opts[:single_threaded], !!opts[:servers]]]
     end
   end
   extend ClassMethods