WELD-2750 Tweak how parent-child creational contexts are created. Use WeakReference to keep track of destroyed instance.

[manovotn]

Attempts to solve a memory leak in dependent instance handling.
See the JIRA (WELD-2750) for detailed description and a reproducer.

First commit:

• Makes sure the set of destroyed instance we keep in each CC is a WeakReference allowing GC to collect them.
• Also changes how we create CC for non-dependent beans when obtaining injectable reference - we no longer unconditionally create child CC as that implies linked them together and potentially holding a reference to an already destroyed instance.

Second commit:

• An improvement on the second step of first commit - tries to avoid creating any superfluous CC when not needed. A side effect is that BeanManagerImpl#getReference can no longer return null.
  • However, after browsing code I didn't see any such usage anyway - CI might still prove me wrong though as I only ran non-container tests locally :)

[manovotn]

CC @ljnelson as this affects the code you recently contributed.

[ljnelson]

Haven't looked in detail but WeakReference was absolutely where I wanted to go initially but didn't think it was worth the complexity. Clearly I was wrong!