HHH-17783, HHH-17918 - Allow early initialization of entity instance by non-owning initializer

[mbladel]

https://hibernate.atlassian.net/browse/HHH-17783
https://hibernate.atlassian.net/browse/HHH-17918

Alternative solution to #8110, including the reproducer kindly provided by @gtoison in #8136.

[mbladel]

For reference: this change allows any entity initializer, even the "non-owning" one, i.e. not the one stored in the EntityHolder, to eagerly execute instance initialization if it gets to it first. This effectively moves the burden of initialization from the owning initializer to the first one that gets to actually initialize the entity. This also means we need some additional checks to avoid duplicating work.

[beikov]

Looks better than #8110. Great work!