[tls] Treat handshake digest algorithm as a session parameter

[tls] Treat handshake digest algorithm as a session parameter

Simplify code by recording the active handshake digest algorithm as a
session parameter.  (Note that we must still accumulate digests for
all supported algorithms, since we don't know which digest will
eventually be used until we receive the Server Hello.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>