Consider making Session implementations public

[marcusdacoregio]

Expected Behavior

It should be possible to extend or delegate to those implementations.

Current Behavior

• SessionRepository implementations are open for extension but cannot be extended, e.g. there is no way to override createSession or save methods.
• This issue mentions that the MongoSession is an internal implementation detail of the MongoIndexedSessionRepository implementation. That actually is not 100% true since MongoSession is returned by the API.
• The AbstractMongoSessionConverter is meant to be extended accordingly to its javadoc, but it is not possible since its abstract methods work with MongoSession and that implementation is package-private
• Since each implementation is part of the public API, it is strange that they are not public

Context

• MongoSession should be public #2217
• How can I extend the save method in the RedisSessionRepository? #2179

[Survival1sm]

#2217 mentions this enhancement would be in 3.3 scheduled for this May, are we still on track to hit this goal?