Tls server name config

[linmaosong2018]

Fixes #384:

• Support TLS SNI by passing in ServerName configuration
• If
  -- InsecureSkipVerify is false
  -- ServerName not configured
  then copy the host part of the connection address as the ServerName. This essentially restores the behavior before ce2275b

Tests done:

• Unit tests - passing
• Manual test with the test code in Replacing tls.DialWithDialer with tls.Client breaks Client Hello #384 - passing

[cbusbey]

👍