Fix mem leak in sslapi test

The recent change in behaviour where you do not get a NewSessionTicket
message sent if you established the connection using a PSK caused a mem
leak to be triggered in sslapitest. It was actually a latent bug and we
were just lucky we never hit it before. The problem is due to complexity
with the way PSK sessions were set up in the early_data tests. PSK session
reference counting was handled differently to normal session reference
counting. This meant there were lots of special cases in the code where
we don't free a session if it is a PSK. It makes things easier if we just
handle PSK reference counts in the same way as other session reference
counts, and then we can remove all of the special case code.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6198)