Reading can return out-of-order timestamps across file boundary in split bags

[emersonknapp]

Description

Individual .db3 files within a split bag use the index to return messages in timestamp order when reading (playing). However, a file is played in full before a next file is loaded. That next file could then return messages with timestamps before the end time of the previous bag, if messages were written out-of-timestamp order across a file boundary.

Old title: Open all sqlite3 files simultaneously (if they are local) to eliminate out-of-order timestamps problem. Changed to reflect the problem, rather than solution space.

Related to #842

[MichaelOrlov]

• Closing as done in the Add support for replaying multiple bags #1848

This issue was resolved by using priority queue sorted by timestamp on upper rosbag2_transport::player level.
We have a separate thread which is reading out next message from rosbag2_cpp::reader and pushing it to the priority queue.
Therefore, the messages will be naturally sorted up to the priority queue size, which is by default 1000 messages.
Reader:read_next() taking care about switching to the next bag split if no more elements in the current bag file.