The class is intended to be inherited by objects that need an SSL context
and a configuration. In practice this will be servers and listeners.
The SSLContext is stored in a rworker_local shared_ptr that makes it
possible to update safely. As the copying is always done behind a lock the
cached local value always holds a valid SSLContext instance for the
duration of all function calls.
Using the pImpl idiom, the routingworker.hh header is not exposed in the
ssl.hh header. This allows the SSLProvider class to be inherited more
easily.
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