Drop entry acl

[ewgRa]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	yes
Deprecations?	no
Tests pass?	-
Fixed tickets	symfony/symfony#2376
License	MIT
Doc PR

As I notice in symfony/symfony#2376, Acl have memory leak.

This is kind of bad design of code, that use something like this:
$this->entry = new Entry(this);

as a result Acl have refcount=2, and second refcount from object itself. As a result when we do unset($acl) it is not free memory at all. This PR - fix for this.

[linaori]

Can you add an UPGRADE-3.0.md file which states exactly which changes are done that break backwards compatibility? This will help people upgrade.

Example: https://github.com/symfony/symfony/blob/master/UPGRADE-3.0.md

[ewgRa]

done.
Also fix another places in code and add test for releaseMemory. Review please

[ewgRa]

@iltar ClassUtils not related to this PR. It was just local quick fix for test run that I accidentally commit and then revert it back. Problem with ClassUtils must be fixed in PR #3

[ewgRa]

2.8 tests are broken now, due to ClassUtils. When I merge your PR about ClassUtils to my PR, all tests passed.