PoC - fix memory leaks

[scyzoryck]

Q	A
Bug fix?	yes
New feature?	no
Doc updated	not yet
BC breaks?	yes
Deprecations?	no
Tests pass?	not yet
Fixed tickets	https://github.com/schmittjoh/serializer/pull/1431 https://github.com/schmittjoh/JMSSerializerBundle/issues/895
License	MIT

PoC of fixing circular references. Any feedback will be appreciated. It contains braking changes.

To do:

• [ ] fix failing tests
• [ ] describe changes in UPGRADING.md

Before

+---------------------+----------+
| benchmark           | mem_peak |
+---------------------+----------+
| XmlMutlipleRunBench | 38.158mb |
| XmlSingleRunBench   | 3.794mb  |
+---------------------+----------+

classDiagram
class Context {
    +VisitorInterface $visitor
    +GraphNavigatorInterface $navigator
}

class GraphNavigator {
    +VisitorInterface $visitor
    +Context $context
}

class AbstractVisitor {
    +GraphNavigatorInterface $navigator
}

GraphNavigator ..> Context
AbstractVisitor ..> Context

AbstractVisitor ..> GraphNavigator
Context ..> GraphNavigator

GraphNavigator ..> AbstractVisitor

After

+---------------------+----------+
| benchmark           | mem_peak |
+---------------------+----------+
| XmlMutlipleRunBench | 3.274mb  |
| XmlSingleRunBench   | 3.272mb  |
+---------------------+----------+

classDiagram
class Context {
    +VisitorInterface $visitor
}

class GraphNavigator {
    +Context $context
}

class AbstractVisitor

AbstractVisitor ..> Context

Context ..> GraphNavigator

[scyzoryck]

Closed for now in favor of https://github.com/schmittjoh/serializer/pull/1473