Automatically run filter plugins on all downloaded files

[msm-code]

This is a draft PR that fixes #281. There are some minor things missing, most importantly:

• [ ] document and/or explain the limitations
• [ ] add the cleanup code (without introducing race conditions)

Your checklist for this pull request

• [x] I've read the contributing guideline.
• [x] I've tested my changes by building and running mquery, and testing changed functionality (if applicable)
• [ ] I've added automated tests for my change (if applicable, optional)
• [ ] I've updated documentation to reflect my change (if applicable)

What is the current behaviour? Filter plugins are not exeuted when downloading files

What is the new behaviour? Filter plugins are applied to downloaded files

Test plan

• enable archive plugin (MQUERY_PLUGINS=plugins.archive:GzipPlugin)
• index a .gz file
• run any query and download the file (it should be decompressed on the fly)

Closing issues

fixes #281