This is used when replica provider call an app contract handler actor.
The app contract handler actor will be called by the long running thread
of replciation when a txn is need to be executed
Fired when replica members has removed by conveyor timeout mechanism.
Call this op to clean up dead or out-of-sync other replicas
Those out-of-sync replicas will be remove after this call
Generate a sync message and then send to other replicas
This is called everytime when a new txn-followup pair is received
by this replica. And, this txn is new to this replica. It will
need to notify other replicas that I received a new txn.
Although this txn may not be new to other replica.
Different from OP_FETCH_HISTORY this will not clean outdated history txns
How many replicas current connect to this replica
If return a empty vec, the replica is still waiting for the second
part of the txn - followup pair.
If this is the second part, the pair is completed, it will return
the tsid as bytes. In this case, please generate the sync message
to other replica, because this replica found a new txn that need
to notify others
Receive sync message from other replica.
Similar to OP_REV_FOLLOWUP, check return value to determin whether or
not to continue call OP_GEN_SYNC_MSG