Before this change, clients were kept alive by making them children of
the TCPServer object. This ownership model is going away (and this was
the only remaining use of it!) so let's just put the clients in a hash
table instead.
This removes the use of StringBuilder::OutputType (which was ByteString,
and only used by the JSON classes). And it removes the StringBuilder
template parameter from the serialization methods; this was only ever
used with StringBuilder, so a template is pretty overkill here.
