net.box: don't block close if called from state machine fiber

Currently, to resubscribe registered watchers net.box installs an
on_connect trigger that writes IPROTO_WATCH requests to the send buffer.
The requests will be sent by the net.box state machine once all
on_connect triggers return. The problem is this trigger may run before a
user-defined trigger that closes the connection, in which case close()
will hang forever, because since #6338, the close method blocks until
the send buffer is emptied, while it can't be emptied in case close is
called by the state machine itself (see #5358).

Let's fix this issue by making close() omit blocking if called from
the state machine fiber.

No user is affected, because IPROTO_WATCH hasn't been officially
released yet so neither changelog nor doc is required.

Closes #6824

NO_CHANGELOG=bug not released
NO_DOC=NO_CHANGELOG