Single state machine across main and object stores

[badrishc]

Changes:

• Share Epoch and StateMachineDriver (SMD) across main and object stores
• Garnet checkpointing uses this shared driver to checkpoint both stores with a single state machine
• Transactions acquiring lockable context ensure that the store version is same across both (main and object) contexts
• Move NumActiveLockingSessions to SMD and ensure correct (v) -> (v+1) switchover with active transactions

Notes:

• Recovery and cluster logic is left unmodified, they assume checkpoints are separate across main and object stores. This allows the PR to remain backward compatible.