This change just gives clear ownership over the CpuBoundWork to the
OutgoingHttpMessage, instead of the previous shared_ptr and weak_ptr
combination with an unclear purpose.
so that `/v1/events` doesn't have to use `IoBoundWorkSlot`.
`IoBoundWorkSlot#~IoBoundWorkSlot()` will wait for a free semaphore slot
which will be almost immediately released by `CpuBoundWork#~CpuBoundWork()`.
Just releasing the already aquired slot manually is more efficient.
This adds generalized IncomingHttpMessage and OutgoingHttpMessage templates
that support different types of streams (via a std::variant) and can both
be used for either requests or responses.
The tacked on metadata from the old HttpRequest and server connection from
the old HttpServerConnection have been moved to HttpApi(Request|Response)
classes that derive from the above generalized message types.