All usages of `AsioTlsStream` were already using `Shared<AsioTlsStream>` to
keep a reference-counted instance. This commit moves the reference counting to
`AsioTlsStream` itself by inheriting from `SharedObject`. This will allow to
implement methods making use of the fact that these objects are
reference-counted.
The changes outside of `lib/base/tlsstream.hpp` are merely replacing
`Shared<AsioTlsStream>::Ptr` with `AsioTlsStream::Ptr` everywhere.
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