Data race in SSLClient

[herolover]

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Testing an app built by clang with thread sanitizer showed a data race in ssl_client.cpp: Imagine that while receiving data, an error occurs, it causes calling Disconnect, that causes calling onReset, where _pimpl is reset. But at the same time a different thread can try send data, using any of Send methods, that calls pimpl() method, that returns a reference, that might be reset in onReset method.