Servants per session (rebased onto develop)



This is the same as gh-2468 but rebased onto develop.

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While looking into an `OverUsageException` issue for @bramalingam, I noticed that the property set in `etc/omero.properties` was not being passed to `ServantHolder`. This fix should simply make changes via `bin/omero config set` propagate. Setting an abnormally low value (e.g. 10) should cause errors quite quickly. Setting to a more reasonable but yet low value (100) might also help find leak issues sooner.