GetTiFlashReplicaRead return corresponding policy string in integer.

GetTiFlashReplicaReadByStr return corresponding policy in string.

AllReplicas means using all the available nodes to do analytic computing, regardless of local zone or other zones.

AllReplicaStr is the string value of AllReplicas.

ClosestAdaptive means using the nodes in the same zone as the entry TiDB.

ClosestAdaptiveStr is the string value of ClosestAdaptive.

ClosestReplicas means using only the nodes in the same zone as the entry TiDB.

ClosestReplicasStr is the string value of ClosestReplicas.

MaxRemoteReadCountPerNodeForClosestReplicas is the max remote read count per node for "closest_replicas".

ReplicaRead is the policy to select TiFlash nodes.