You now cannot get an unconnected LibIMAP::Client, but you can still
close it. This makes for a nicer API where we don't have a Client object
in a limbo state between being constructed and being connected.
This code still isn't as nice as it should be, as TLS::TLSv12 is still
not a Core::Stream::Socket subclass, which would allow for consolidating
most of the TLS/non-TLS code into a single implementation.