Fix serialisation of uninitialised PersistentCollection instances

[alcaeus]

Q	A
Type	bug
BC Break	no
Fixed issues	Fixes #1864, Replaces #1914

Summary

This ensures that any internal data from PersistentCollection instances are serialised to have a usable collection instance after serialisation. This affects the mongoData and snapshot properties along with isDirty and hints.

Ping @watari

[malarzm]

I still think that using unserialised PersistentCollection is a can of worms but this is better than #1914 :)

[watari]

Hi there. Sorry for delay. First of all - thank you for PR with fix :)
But still PersistentCollection is unusable after serialization since It does not have link to UoW and cannot perform hydration of raw data. I agree with @malarzm , that PersistentCollection should not be serialized since it will be broken after unserialization.

I propose just to forget about this issue until it do not brake anything :)

About my PR - it was bad in general and I did not count references. In such way I fixed it for my projects and it was worked for me.
I think that this issue will be really actual when implementation of results caching in lib will begin.

[malarzm]

@watari for the record, unserialised collection will have UnitOfWork if you call ->setDocumentManager()