Improve LOGGER levels in Memoizer

Various conditions in Memoizer were being logged
at DEBUG when they were really error or at least
exceptional states. This corrects most of them.
There is still some question of what should be
at TRACE and what at DEBUG.

Also, one log statement was being reached if the
condition didn't hold. This is now checked by an
if/else.

See:
https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7548
Update version history for 5.0.3
Update expected release date for 5.0.3
Merge pull request #1250 from joshmoore/rebased/develop/memo-warnings

Improve LOGGER levels in Memoizer (rebased onto develop)
Merge pull request #1253 from melissalinkert/rebased/develop/version-history

Update version history for 5.0.3 (rebased onto develop)
Update auto-generated documentation

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