Refactor session store implementation

[tharindud]

• Current implementation of sessions is located in the core. This includes the in-memory and Redis implementations.
• Also runtime initialization uses an enumeration to select the session store type.

Proposed Change

• Factor out the Redis implementation of the session store out to a new module so that we can remove the Redis dependency from the core.
• Runtime initialization should be possible with an externally injected session store instance instead of a enumeration specifying the session store type. This would allow easier extension of session stores.

[tharindud]

The changes to Runtime and the removal of MemoryType enum has already been done as part of the Runtime singleton fixes.