Create a new builtin_msgs/SteadyTime

[tfoote]

This is a follow up to #94 splitting off the open ended design component.

The builtin_msgs/Time is specifically ROS Time. We have discussed making a separate time type for Steady Time.

Since there can be multiple steady time sources on the system it would be good to consider capturing the epoch from any source of steady time. Likely a UUID of some sort that can be leveraged to make sure that a steady time point is only allowed to be compared to another steady time point on the same epoch.

As a strawman I'd suggest something like

SteadyTime.msg

int32 sec
uint32 nsec
unique_identifier_msgs/UUID epoch