[rospy] fix param cache when node is subscribing and setting a parameter

[mgrrx]

If one ROS node is setting and at the same time subscribing to the same parameter the ROS master does not notify about a parameter change. This change makes sure that the cache is updated after a set_param call.

Before

In [1]: import rospy
In [2]: rospy.init_node('asdf', log_level=rospy.DEBUG)
In [3]: rospy.set_param('a', 5)
In [4]: rospy.get_param('a')
Out[4]: 5
In [5]: rospy.get_param_cached('a')
Out[5]: 5
In [6]: rospy.set_param('a', 10)
In [7]: rospy.get_param_cached('a')
Out[7]: 5
In [8]: rospy.get_param('a')
Out[8]: 5

After

In [1]: import rospy
In [2]: rospy.init_node('asdf', log_level=rospy.DEBUG)
In [3]: rospy.set_param("a", 5)
In [4]: rospy.get_param("a")
Out[4]: 5
In [5]: rospy.get_param_cached("a")
Out[5]: 5
In [6]: rospy.set_param("a", 10)
In [7]: rospy.get_param("a")
Out[7]: 10
In [8]: rospy.get_param_cached("a")
Out[8]: 10

[dirk-thomas]

Thanks for the patch.