Skip nodes with unresolved address during initialization

[mxmsk]

This change fixes several issues:

1. Major issue: all addresses passed to the NewCluster must be resolvable. This breaks the ability to include a future nodes in configuration that could simplify cluster migrations.
2. NewCluster saves nodes by using provided addresses but then the existing Cluster manages them by connection addresses (dns-resolved ip's). This lead to unnecessary reconnection on first updateMappings call.
3. When new node is discovered from CLUSTER SLOTS it is added with connection address, that potentially breaks TLS, because server name become inaccessible. To fix, redis setup should provide "hostname" with cluster slots metadata.
4. Minor issue: NewCluster checked initialized nodes in master map while adding them to the nodes map. This could lead to unnecessary connections if address has duplicates.

[g7r]

I'd suggest using net.JoinHostPort here and on addrStr := ... line.