ephy-shell: do not lose a remote startup context

When starting epiphany multiple times in quick succession, we would
asynchronously call ephy_session_resume() from ephy_shell_activate(),
but ephy_session_resume() might not finish until after a subsequent call
to ephy_shell_activate(), causing us to lose the commands from a remote
instance and execute another's commands multiple times. Instead,
distinguish between local and remote startup contexts so that
ephy_shell_activate() can know whether we are receiving commands from a
remote instance, and not resume the session if so. This allows us to
synchronously load the commands before the next call to
ephy_shell_before_emit() invalidates the remote startup context.

https://bugzilla.gnome.org/show_bug.cgi?id=735577