* Fix virtual call in ClientImpl::~ClientImpl()

This fixes a warning in clang tidy:

> Call to virtual method 'ClientImpl::shutdown_ssl' during
> destruction bypasses virtual dispatch

ClientImpl::~ClientImpl() calls lock_socket_and_shutdown_and_close()
that itself calls shutdown_ssl().  However, shutdown_ssl() is virtual
and C++ does not perform virtual dispatch in destructors, which results
in the wrong overload being called.

This change adds a non-virtual shutdown_ssl_impl() function that is
called from ~SSLClient().  We also inline sock_socket_and_shutdown_and_close()
and removes the virtual call in ~ClientImpl().

* Inline and remove lock_socket_and_shutdown_and_close()

The function only has one caller.