tls: fix lazy initialization of clienthello parser

`server.SNICallback` was initialized with `SNICallback.bind(this)`, and
therefore check `this.SNICallback === SNICallback` was always false, and
`_tls_wrap.js` always thought that it was a custom callback instead of
default one. Which in turn was causing clienthello parser to be enabled
regardless of presence of SNI contexts.

/cc @bnoordhuis