creation of client TLS ctx before exclusive mode

creation of client TLS ctx before exclusive mode

When the server is configured (inside `apply_configuration`) a client
TLS context cache is created and attached to the global server object.
It is then used by `configure_view` flow (and also during runtime though
the zone manager).

It is now created before the exclusive mode, and the swap of the
previous TLS cache ctx is done at the end of the exclusive mode, if
everything went well.

This allows us (among other follow-up changes) to move the
`configure_views` function outside of the exclusive mode.