Test added for the "retain" recovery strategy. This strategy makes sure
a full history of index changes is made so that if the Ledger is wiped
out, the Ledger cna be fully rebuilt from the Journal.
This exposed two journal compaction problems
- The BestRun selected did not have the source files correctly sorted in
order before compaction
- The compaction process incorrectly dealt with the KeyDelta object
left after a compaction - i.e. compacting twice the same key caused that
key history to be lost.
These issues have now been corrected.
