When classifying a client's selection of TLS ciphers, if the client
ciphers are not yet available, do not cache the result. Previously,
we had cached the unavailability of the cipher list and never looked
again, which in turn led us to assume that the client only supported
the ancient V1 link protocol.  This, in turn, was causing Stem
integration tests to stall in some cases.  Fixes bug 30021; bugfix
on 0.2.4.8-alpha.