caddytls: Fix sni_regexp matcher to obtain layer4 contexts

[vnxme]

This PR is the mainline part of the fix described here for the issue with missing {tls.regexp.*} placeholders reported here.

[mholt]

Do you think we could refactor this and simplify it a bit by first trying to get a context, and then if we do, THEN get the replacer from it?

i.e. something like:

var ctx context.Context
// try to set ctx
// if ctx is not nil, try to get replacer
// if replacer is nil, just use NewReplacer

[mholt]

Looks good!! Thanks!