keep connection header information in rosbag operations (#679, #685)

[ddimarco]

This PR provides a fix for the issue that rosbag filter & rosbag compress removes latching information from topic connection headers.
See #679 for a nice description. This PR is based on that discussion, but follows a slightly different implementation.

You can see the difference by using this script (the script needs the patched code to access the bag headers):

# using current kinetic-devel branch
$ rosbag compress test_rosbag.bag --output-dir /tmp/
$ show_bag_headers.py /tmp/test_rosbag.bag           
/chatter_no_latch: {u'topic': '/chatter_no_latch', u'message_definition': 'string data\n', u'md5sum': '992ce8a1687cec8c8bd883ec73ca41d1', u'type': 'std_msgs/String'}
/chatter_latch: {u'topic': '/chatter_latch', u'message_definition': 'string data\n', u'md5sum': '992ce8a1687cec8c8bd883ec73ca41d1', u'type': 'std_msgs/String'}

# this PR
$ rosbag compress test_rosbag.bag --output-dir /tmp/
$ show_bag_headers.py /tmp/test_rosbag.bag           
/chatter_no_latch: {u'message_definition': 'string data\n', u'callerid': '/talker', u'latching': '0', u'md5sum': '992ce8a1687cec8c8bd883ec73ca41d1', u'topic': '/chatter_no_latch', u'type': 'std_msgs/String'}
/chatter_latch: {u'message_definition': 'string data\n', u'callerid': '/talker', u'latching': '1', u'md5sum': '992ce8a1687cec8c8bd883ec73ca41d1', u'topic': '/chatter_latch', u'type': 'std_msgs/String'}

[mikepurvis]

Looks like a good change; I've rebased it onto melodic-devel. 👍

[mikepurvis]

Remaining failure on Bionic is a known-flaky test. LGTM.

[plusk01]

Will this be seen in kinetic, or just melodic?

[dirk-thomas]

There is no reason to break the API signature here. The new argument should be made optional. See first commit on #1473.