Add option to control if remote replicas should be used

Previously the driver would always failover to use replicas
in the remote datacenters if there was no replicas available
in local_dc when using DC/RackAwareRoundRobinPolicy. There was
no control over it. This is not how the other driver handle it.

This commit adds a option to dc/rackAwareRR to control if dc
failover is permitted. The default stays the same (use remote
datacenter).

policies.go

@@ -948,19 +948,37 @@ func (host selectedHostPoolHost) Mark(err error) {
 }
 
 type dcAwareRR struct {
-	local           string
-	localHosts      cowHostList
-	remoteHosts     cowHostList
-	lastUsedHostIdx uint64
+	local            string
+	localHosts       cowHostList
+	remoteHosts      cowHostList
+	lastUsedHostIdx  uint64
+	permitDCFailover bool
+}
+
+type dcFailoverDisabledPolicy interface {
+	setDCFailoverDisabled()
+}
+
+type dcAwarePolicyOption func(p dcFailoverDisabledPolicy)
+
+func HostPolicyOptionDisableDCFailover(p dcFailoverDisabledPolicy) {
+	p.setDCFailoverDisabled()
 }
 
 // DCAwareRoundRobinPolicy is a host selection policies which will prioritize and
 // return hosts which are in the local datacentre before returning hosts in all
 // other datercentres
-func DCAwareRoundRobinPolicy(localDC string) HostSelectionPolicy {
-	return &dcAwareRR{local: localDC}
+func DCAwareRoundRobinPolicy(localDC string, opts ...dcAwarePolicyOption) HostSelectionPolicy {
+	p := &dcAwareRR{local: localDC, permitDCFailover: true}
+	for _, opt := range opts {
+		opt(p)
+	}
+	return p
 }
 
+func (d *dcAwareRR) setDCFailoverDisabled() {
+	d.permitDCFailover = false
+}
 func (d *dcAwareRR) Init(*Session)                       {}
 func (d *dcAwareRR) Reset()                              {}
 func (d *dcAwareRR) KeyspaceChanged(KeyspaceUpdateEvent) {}
@@ -1035,7 +1053,12 @@ func roundRobbin(shift int, hosts ...[]*HostInfo) NextHost {
 
 func (d *dcAwareRR) Pick(q ExecutableQuery) NextHost {
 	nextStartOffset := atomic.AddUint64(&d.lastUsedHostIdx, 1)
-	return roundRobbin(int(nextStartOffset), d.localHosts.get(), d.remoteHosts.get())
+	if d.permitDCFailover {
+		return roundRobbin(int(nextStartOffset), d.localHosts.get(), d.remoteHosts.get())
+	} else {
+		return roundRobbin(int(nextStartOffset), d.localHosts.get())
+	}
+
 }
 
 // RackAwareRoundRobinPolicy is a host selection policies which will prioritize and
@@ -1047,15 +1070,19 @@ type rackAwareRR struct {
 	// It is accessed atomically and needs to be aligned to 64 bits, so we
 	// keep it first in the struct. Do not move it or add new struct members
 	// before it.
-	lastUsedHostIdx uint64
-	localDC         string
-	localRack       string
-	hosts           []cowHostList
+	lastUsedHostIdx  uint64
+	localDC          string
+	localRack        string
+	hosts            []cowHostList
+	permitDCFailover bool
 }
 
-func RackAwareRoundRobinPolicy(localDC string, localRack string) HostSelectionPolicy {
-	hosts := make([]cowHostList, 3)
-	return &rackAwareRR{localDC: localDC, localRack: localRack, hosts: hosts}
+func RackAwareRoundRobinPolicy(localDC string, localRack string, opts ...dcAwarePolicyOption) HostSelectionPolicy {
+	p := &rackAwareRR{localDC: localDC, localRack: localRack, hosts: make([]cowHostList, 3), permitDCFailover: true}
+	for _, opt := range opts {
+		opt(p)
+	}
+	return p
 }
 
 func (d *rackAwareRR) Init(*Session)                       {}
@@ -1067,6 +1094,10 @@ func (d *rackAwareRR) MaxHostTier() uint {
 	return 2
 }
 
+func (d *rackAwareRR) setDCFailoverDisabled() {
+	d.permitDCFailover = false
+}
+
 // Experimental, this interface and use may change
 func (d *rackAwareRR) SetTablets(tablets []*TabletInfo) {}
 
@@ -1101,7 +1132,12 @@ func (d *rackAwareRR) HostDown(host *HostInfo) { d.RemoveHost(host) }
 
 func (d *rackAwareRR) Pick(q ExecutableQuery) NextHost {
 	nextStartOffset := atomic.AddUint64(&d.lastUsedHostIdx, 1)
-	return roundRobbin(int(nextStartOffset), d.hosts[0].get(), d.hosts[1].get(), d.hosts[2].get())
+	if d.permitDCFailover {
+		return roundRobbin(int(nextStartOffset), d.hosts[0].get(), d.hosts[1].get(), d.hosts[2].get())
+	} else {
+		return roundRobbin(int(nextStartOffset), d.hosts[0].get(), d.hosts[1].get())
+	}
+
 }
 
 // ReadyPolicy defines a policy for when a HostSelectionPolicy can be used. After