Fix startTime and endTime computation when chunks are not sorted

[pankdm]

From reading discussion ros/ros_comm#952 about python implementation this seems like a similar bug on rosbag.js. Fixing this by iterating through all chunks not just first and last.

Note: I used the following script to create fixture bag (reference):

from rosbag import Bag
from rospy import Time
from std_msgs.msg import String

b = Bag("example-unsorted-chunks.bag", 'w')
b.chunk_threshold = 0
b.write("/foo", String("2"), Time(2))
b.write("/foo", String("3"), Time(3))
b.write("/foo", String("1"), Time(1))
b.close()

Double checking created bag (note rosbag doesn't show proper duration either):

$ rosbag info example-unsorted-chunks.bag
path:        example-unsorted-chunks.bag
version:     2.0
duration:    -1.0s
start:       Jan 01 1970 00:00:02.00 (2.00)
end:         Jan 01 1970 00:00:01.00 (1.00)
size:        5.2 KB
messages:    3
compression: none [3/3 chunks]
types:       std_msgs/String [992ce8a1687cec8c8bd883ec73ca41d1]
topics:      /foo   3 msgs    : std_msgs/String

Test Plan

• Added a new test to cover this
• yarn run test

[MatthewSteel]

Yikes. Thanks for the link to the ros_comm discussion.

Seems possible that some "get messages" calls won't return time-ordered messages when this happens, but this seems like a good improvement anyway: cheap, short, and captures most of the upside -- with the wrong start/end times users might not see some messages at all, which is a lot worse than seeing messages in the wrong order.

[pankdm]

Yep, exactly! Let's do this small incremental improvement first and then we can follow up on the ordering if needed.