encapsulates all the connection specific state for a HTTP/S connection
one of these is hung from the selector attachment and is used to locate
everything from that.
HttpContext represents a mapping between a protocol (http or https) together with a root URI path
to a
HttpHandler which is invoked to handle requests destined
for the protocol/path on the associated HttpServer.
a (filter) input stream which can tell us if bytes are "left over"
on the underlying stream which can be read (without blocking)
on another instance of this class.
The class can also report if all bytes "expected" to be read
were read, by the time close() was called.