tls: add tlsSni option - closes #1114

[alexandre-abrioux]

Hi everyone,

This PR introduces a new boolean option tlsSni that will basically, when enabled, copy the Redis hostname to tls.servername before the connection initialization,
allowing each node of a Redis Cluster to possess its own SNI.

This PR has been opened following the issue #1114

It is also related to #948

Thank you for your time!

[alexandre-abrioux]

Added a test for cluster connections.

For clusers I had to copy the options.host value to options.hostOriginal before it gets processed by the dnsLookup function, so that the original hostname could get passed to the tls.servername option instead of its IP.

It should be ready to be reviewed now.
Thank you!

[alexandre-abrioux]

changed fork branch, PR moved here:
#1137