Deprecate common::Time?

[chapulina]

With C++11, we could deprecate common::Time in favor of std::chrono.

That would impact some other libraries' public APIs, like ign-sensors.

[ahcorde]

• ign-math Added functions to convert between time_point and secNsec gz-math#150
• ign-gui Using std::chrono instead of common::Time gz-gui#116
  • This one depends on ign-math PR
• ign-common Deprecated common:Time #90
  • Deprecated common::Time constructor and static functions
• ign-msgs Added utility functions to convert between msg::time and std::chrono time_point gz-msgs#81
  • This one depends on ign-math PR
• ign-gazebo Replaced common::Time for std::chrono gz-sim#309
  • This one depends on ign-math PR
• ign-sensors Replaced common::Time for std::chrono gz-sensors#41
  • This one depends on ign-msgs PR
• ign-rendering Replace common::Time with std::chrono gz-rendering#135
  • This one depends on ign-math PR