wait_for_server should use walltime

[beetleskin]

wait_for_server() uses ros-time for checking how much time passed. This has two issues when using sim time

• the function never returns when /clock is never published, which is unexpected
• the function returns too early, if time gets initialized while wait_for_service() is running

The function should behave like its bigger brother wait_for_service().

This was already addressed in #19, and fixed in https://github.com/esteve/actionlib/commit/3dd09c548afb0391e0feddb4025a6d77105f70b5. We need to port this fix onto the current master branch.

See also:

• https://answers.ros.org/question/10290/weird-behavior-when-instantiating-simpleactionclient-right-after-initializing-node-in-rospy/