Check a system property before EntityManager injection outside work unit happens

[nuzayats]

There are many folks who complains about implicit EntityManager injection because it very likely to bring EntityManager leakage:

#739
#730

I think it's reasonable to check a system property before implicit injection happens. it has no compatibility breakage, and simple.

Inspired by workaround of this: https://issues.apache.org/jira/browse/COLLECTIONS-580

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@sameb
Hi, I guess you might thought this PR wouldn't be a correct solution to fix this issue. If so it's fine but anyway I believe the issue which this PR addressed should be fixed.

Why don't you make injection process of EntityManager to throw Exception when it happens outside unit of work as the message in Exception states? Do you afraid of compatibility breakage?

Any feedback or thoughts greatly appreciated :)