crypto/tls: add server side SNI support.

crypto/tls: add server side SNI support.

With this in place, a TLS server is capable of selecting the correct
certificate based on the client's ServerNameIndication extension.

The need to call Config.BuildNameToCertificate is unfortunate, but
adding a sync.Once to the Config structure made it uncopyable and I
felt that was too high a price to pay. Parsing the leaf certificates
in each handshake was too inefficient to consider.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5151048