implement caching for parameter model

[NickZ]

rospy currently has a bug which makes it check the parameter server every time the node publishes to rosout. While upstream has been notified, this may not make it into rosdistro for a while, and this is a performance issue. To work around this, we should implement a caching layer for the parameter server.

The cache should not populate by default, and should only cache documents when the number of accesses reaches a certain threshold. Once this threshold has been reached, the document should be cached and watch() should be run on it. watch() triggers an event, and this should be used to update the cache if the document gets updated.

[sevenbitbyte]

@NickZ do we have a bug number for the upstream issue?

[NickZ]

The upstream issue can be found here: ros/ros_comm#1575