Add support for flush mode to MongoDB session repositories

[vpavic]

Since MongoDB support has been migrated back to this repository, I'm cloning the issue originally reported in spring-projects/spring-session-data-mongodb#107.

Original issue description:

Immediate FlushMode can be used as a strategy for dealing with race conditions, by writing changes to data store as they happen, vs traditionally on invocation of #save on session repository.

Note that this is an option only for Servlet-based implementations, as org.springframework.session.Session#setAttribute returns void (and not Mono<Void>).

All SessionRepository implementations in Spring Session core modules support this and could be used for inspiration.