Session tickets should never be enabled with the currect code on both
clients and servers. Set the GNUTLS_NO_TICKETS flag when opening a TLS
session in case this understanding is wrong, or it changes in future, to
reduce the TLS attack surface.
It seems gnutls (at least in version 3.6.14) allows clients to connect
using TLS1.2 when it has a DTLS version enabled in the priority cache.
Disable all DTLS versions in order to disable TLS1.2.
Add more comments and assertions, replace getsockopt() call with
SCK_GetIntOption(), replace strncmp() with memcmp(), move a return
statement for clarity, and remove an unused field from the instance
record.
Add "nocerttimecheck" directive to specify the number of clock updates
that need to be made before the time validation of certificates is
enabled. This makes NTS usable on machines that don't have a RTC.
Make the NTS-KE retry interval exponentially increasing, using a factor
provided by the NKE session. Use shorter intervals when the server is
refusing TCP connections or the connection is closed or timing out
before the TLS handshake.
The server session instances are reused for different clients. Separate
the server name from the label used in log messages and set it on each
start of the session.
