Improve progressbar ETA

[xshill]

Currently, we create a progressbar that counts all the messages we need to parse. This is not very accurate because most messages take a very short time to parse, but images take a long time. Since we have a lot of fast messages, this makes the estimated time unreliable (probably shorter than it should be).

If we filtered for only messages that take a long time to parse when creating the progressbar, we could provide a more accurate ETA.

[alxbl]

Keep in mind #315 when addressing this. A big problem we have for larger captures is attempting to slurp the entire session file into memory and causing out of memory crashes. I have a branch on my fork which attempts to address that, but there are a few more things I want to do before opening a PR with the changes.