Make RealmEntityManagerFactory thread-safe

[adutra]

Description

RealmEntityManagerFactory is an application-scoped bean and as such is expected to be accessed by multiple threads concurrently; but its getOrCreateEntityManager method is not thread-safe: it exhibits a non-synchronized check-then-act sequence involving a non-thread-safe map implementation.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Documentation update
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Manually as it is difficult to bring evidence of thread-safety issues in a unit test.

Checklist:

Please delete options that are not relevant.

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• If adding new functionality, I have discussed my implementation with the community using the linked GitHub issue

[snazy]

Nice catch!