- client-side StoresSessions trait which implements
an opaque bytestring->bytestring map.
- persistence message layer which reuses the existing types to
define a key and value format for saving sessions.
- second handshake route if the server agrees to resume.
- add 'features' tests, to check specific features work at the
integration level.
Testing this is relatively annoying because OpenSSL
don't do error handling as specified by the RFC,
and because the RFC contradicts itself. Quality
all round.