REP-6088 Fix display of verification summary

[FGasper]

PR #117 introduced some bugs in the verification summary logging. This addresses the following:

• Mismatches were previously shown in an indeterminate order. Now they’re consistently sorted by the mismatched documents’ _id.
• Documents with missing fields were being logged as entirely missing. That logic is corrected here.
• The table of missing/changed documents previously iterated through the _ids persisted in the task rather than the actual missing/changed documents. Thus, if there were only a handful of missing documents in a recheck task that contained thousands of document IDs, all of that task’s document IDs would be logged as missing. This was an oversight from PR REP-6088 Tolerate high numbers of mismatches #117, which should have updated the logic to build that table as it migrated that for the mismatched-documents table. This changeset does the necessary update.

Additional changes here:

• build.sh now logs the git commit & time that it’s building into the binary.
• The compareOneDocument() method is moved to compare.go, which more sensible since that’s where it’s called.