Speed up indexed iterator on negative search by topic

[wkalt]

Prior to this commit, the indexed iterator did not rule out chunks based on topic specification, which meant it still opened chunks when no topics would match. With this change the tool does less IO when there is no matching data.

[wkalt]

timings:

[~/repos/mcap/go/cli/mcap] (main) $ time mcap cat ~/data/bags/cal_loop.mcap --topics /foobar

real    0m0.969s
user    0m0.911s
sys     0m0.163s
[~/repos/mcap/go/cli/mcap] (main) $ git checkout -
Switched to branch 'indexed-iterator-negative-lookup'
[~/repos/mcap/go/cli/mcap] (indexed-iterator-negative-lookup) $ go install
[~/repos/mcap/go/cli/mcap] (indexed-iterator-negative-lookup) $ time mcap cat ~/data/bags/cal_loop.mcap --topics /foobar

real    0m0.157s
user    0m0.227s
sys     0m0.024s

[james-rms]

Right now this reader works with files with chunk indexes but no message indexes, which is a case I'd like to continue to support. You could check if the chunk index has any message index offsets before ruling topics out based on their absence.