Add public Network.close() method to perform controlled network shutdown

[jagerman]

This renames the close_gracefully() method to just close() and makes
it public so that an application has a bit more control over Network
shutdown.

libsession-util ran into an issue where the quic::Network destruction
segfaults because it has callbacks (e.g. connection closing) that expect
to be able to use their local quic::Network instance, but can't
because it's in the process of being destroyed.

Essentially the pattern is this:

class A {
    quic::Network net;
};
A a;
// start a connection with a connection_close callback that
// references `a`.

If the quic::Network destruction is triggered by the destruction of a,
then the quic::Network destructor is firing after a is no longer in
a valid state, and so the callback referencing a is invalid.

Exposing close() allows the application to deal with this: it can add a
net.close() either in A's destructor or some other shutdown code so
that it can ensure the quic Network shutdown happens before a
becomes unusable.

This also adds a non-synchronous call_soon() to mirror the call(ep)
and call_soon(ep) methods for shutdown down individual endpoints.