Make DB_CLOSE_ON_EXIT safer

[katzyn]

DB_CLOSE_ON_EXIT is one of the most popular misused dangerous settings.

1. Default value of this setting is now FALSE for in-memory databases to avoid registration of shutdown hook. I'm not sure about that, but I think we don't really need to explicitly close any in-memory databases on VM shutdown. Anyway, it is possible to restore this feature with DB_CLOSE_ON_EXIT=TRUE.
2. For persistent databases the default value is still TRUE as it should, but if it was set to FALSE explicitly, the shutdown hook is registered anyway and VM shutdown event executes a CHECKPOINT command to flush data on disk for more safety.
3. Documentation of this setting now has all necessary warnings and explains how to use it properly.

@grandinj

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[grandinj]

LGTM