tls: scope loop vars with let

`lib/_tls_common.js` had instances of `for` loops that defined variables
with `var` such that they were re-declared in the same scope. This
change scopes those variables with `let` so that they are not
re-declared.

PR-URL: https://github.com/nodejs/node/pull/4853


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>