`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.
It shouldn't ignore it!
There're two possibile cases, which should be handled properly:
1. Having a default `SNICallback` which is using contexts, added with
`server.addContext(...)` routine
2. Having a custom `SNICallback`.
In first case we may want to opt-out setting `.onsniselect` method (and
thus save some CPU time), if there're no contexts added. But, if custom
`SNICallback` is used, `.onsniselect` should always be set, because
server contexts don't affect it.
Split `tls.js` into `_tls_legacy.js`, containing legacy
`createSecurePair` API, and `_tls_wrap.js` containing new code based on
`tls_wrap` binding.
Remove tests that are no longer useful/valid.