Added support for SSL Server Name Indication.

Motivation:

When we need to host multiple server name with a single IP, it requires
the server to support Server Name Indication extension to serve clients
with proper certificate. So the SniHandler will host multiple
SslContext(s) and append SslHandler for requested hostname.

Modification:

* Added SniHandler to host multiple certifications in a single server
* Test case

Result:

User could use SniHandler to host multiple certifcates at a time.
It's server-side only.