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.

Calling `AsioTlsStream::async_shutdown()` performs a TLS shutdown which
exchanges messages (that's why it takes a `yield_context`) and thus has the
potential to block the coroutine. Therefore, it should be protected with a
timeout. As `async_shutdown()` doesn't simply take a timeout, this has to be
implemented using a timer. So far, these timers are scattered throughout the
codebase with some places missing them entirely. This commit adds helper
functions to properly shutdown a TLS connection with a single function call.

This new helper functions allows deduplicating the timeout handling for
`async_shutdown()`.

This new helper function has proper timeout handling which was missing here.

The reason for introducing AsioTlsStream::GracefulDisconnect() was to handle
the TLS shutdown properly with a timeout since it involves a timeout. However,
the implementation of this timeout involves spwaning coroutines which are
redundant in some cases. This commit adds comments to the remaining calls of
async_shutdown() stating why calling it is safe in these places.