chore: add multiplexed session implementations for CachedSession/Sess…

…ionFuture interfaces. (#2973)

* chore: add multiplexed session implementations for CachedSession/SessionFuture  interfaces.

* chore: add comments.

* chore: add session replacement handler for multiplexed session.

* chore: address comments.

* chore: fix comments.

* chore: fix comments.