Defer closing of connection when it is the last connection open to a

Defer closing of connection when it is the last connection open to a
particular server.  This prevents a pattern of opening and closing
that occurs when IMAIL starts up (due to probes performed prior to
opening the folder).  It also allows the user to do completion in the
folder space without occurring the same penalty.  The downside is that
the connection remains open for about a minute after the user "closes"
it, which will be remedied in a future revision.