BUG/MINOR: ssl: properly ref-count the tls_keys entries

BUG/MINOR: ssl: properly ref-count the tls_keys entries

Commit 200b0fa ("MEDIUM: Add support for updating TLS ticket keys via
socket") introduced support for updating TLS ticket keys from the CLI,
but missed a small corner case : if multiple bind lines reference the
same tls_keys file, the same reference is used (as expected), but during
the clean shutdown, it will lead to a double free when destroying the
bind_conf contexts since none of the lines knows if others still use
it. The impact is very low however, mostly a core and/or a message in
the system's log upon old process termination.

Let's introduce some basic refcounting to prevent this from happening,
so that only the last bind_conf frees it.

Thanks to Janusz Dziemidowicz and Thierry Fournier for both reporting
the same issue with an easy reproducer.

This fix needs to be backported from 1.6 to 1.8.