add DCLocalRoundRobin host selection policy

[isopov]

As requested in #956 we need an option to not fallback to remote datacenter in DCAware host selection policy.
Unfortunately we first try to connect to remote datacenter host and receive local hosts there. So the way with HostFilter does not work for us. It seems that such host selection policy should work for us...

[joao-r-reis]

Adding a whole new type for this seems unnecessary, we just need a new boolean field on the existing policy like localOnly.

Also, there's nothing in the integration test suite that verifies that the driver will be able to successfully connect and initialize with the scenario you're describing. Unfortunately this would require a cluster with two datacenters and the cluster we create for CI only has one datacenter. Maybe we could change the CI cluster to have two datacenters though.

[isopov]

In my app I finished not with fork of this driver, but with this custom selection policy:

func wrapForceDC(internal gocql.HostSelectionPolicy, dcLocal string) gocql.HostSelectionPolicy {
	return &forceDCPolicy{
		dc:       dcLocal,
		internal: internal,
	}
}

type forceDCPolicy struct {
	dc       string
	internal gocql.HostSelectionPolicy
}

func (f *forceDCPolicy) AddHost(host *gocql.HostInfo) {
	if f.IsLocal(host) {
		f.internal.AddHost(host)
	}
}

func (f *forceDCPolicy) RemoveHost(host *gocql.HostInfo) {
	if f.IsLocal(host) {
		f.internal.RemoveHost(host)
	}
}

func (f *forceDCPolicy) HostUp(host *gocql.HostInfo) {
	if f.IsLocal(host) {
		f.internal.HostUp(host)
	}
}

func (f *forceDCPolicy) HostDown(host *gocql.HostInfo) {
	if f.IsLocal(host) {
		f.internal.HostDown(host)
	}
}

func (f *forceDCPolicy) Init(s *gocql.Session) {
	f.internal.Init(s)
}
func (f *forceDCPolicy) KeyspaceChanged(e gocql.KeyspaceUpdateEvent) {
	f.internal.KeyspaceChanged(e)
}
func (f *forceDCPolicy) SetPartitioner(p string) {
	f.internal.SetPartitioner(p)
}

func (f *forceDCPolicy) IsLocal(host *gocql.HostInfo) bool {
	return host.DataCenter() == f.dc
}

func (f *forceDCPolicy) Pick(query gocql.ExecutableQuery) gocql.NextHost {
	return f.internal.Pick(query)
}

And use it like this:

wrapForceDC(gocql.TokenAwareHostPolicy(gocql.RoundRobinHostPolicy(), gocql.ShuffleReplicas()), dcLocal)

[isopov]

Also, there's nothing in the integration test suite that verifies that the driver will be able to successfully connect and initialize with the scenario you're describing

We already have code dealing with several datacenters and fallback to not preferred one and no tests for it