Refactor *Close into AbstractCloseableAmdServant (Fix #11074)

Changing the signature of preClose and postClose in
AbstractAmdServant to include an Ice.Current argument
(See 94fca87e891185b6ca4256512ef16812396d29df) left
ExporterI with an uncalled preClose method which in turn
left temp files dangling.

Adding `@Override` to the protected method would prevent
such errors in the future, but this refactoring goes a
step further and makes the methods abstract so that they
must be implemented.
Add OMERO_TEMPDIR to diagnostics output (See #11074)
Syntactic adjustments from @mtbc
Merge pull request #1260 from joshmoore/11074-export-close

11074 export close