xmlquery: use compressed length when available

[ato]

Description

Use the compressed length when it's available in XmlQueryIndexSource.

The field is confusingly misnamed compressedendoffset in the XML but OpenWayback and consequently OutbackCDX actually use this for the "S" CDX field (compressed length).

Motivation and Context

Without this field when WARC files are accessed over HTTP pywb will make open byte range requests which results in a lot more data being read from disk than necessary.

CC @anjackson

Screenshots (if appropriate):

N/A

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed. (test_force_https failures seem unrelated. I get the same failures with the master branch prior to this change.)

[ikreymer]

Makes sense, I didn't know this field existed in XmlQueryIndexSource. As long as its safe to assume that compressedendoffset is always the length, and not the end offset.