The MainContext that miscellaneous session-related asynchronous callbacks are invoked on.
(Eg, setting idle_timeout will add a timeout source on this context.)
For a plain Session, this property is always set to the
MainContext that is the thread-default at the time the session was created, and
cannot be overridden. For the deprecated Session subclasses, the default value is
null, meaning to use the global default MainContext.
If use_thread_context is
false, this context will
also be used for asynchronous HTTP I/O.