Enforce that the current realm is connected while running GATT proced…

…ures.

This change makes it so that a disconnect() call takes effect immediately and
prevents any subsequent changes to member variables from a returning GATT
procedure, even if the GATT procedure started before the disconnection. It also
forces all active requests to fail with a NetworkError so that no remnants of
the connection keep running after it's terminated.

This reverses the readValue() promise and event order from #175, in order to
avoid needing to perform a microtask checkpoint inside the algorithm.

Fixes #114. Fixes #171.