set X-CSTP-{Lease-Duration,Session-Timeout,Session-Timeout-Remaining} headers as Cisco servers appear to set them

Old Cisco docs which don't appear to accurately describe the behavior of Session-Timeout or Lease-Duration, but do identify their maximum values as 1209600 seconds (14 days): https://www.cisco.com/assets/sol/sb/RV345P_Emulators/RV345P_Emulator_v1-0-01-17/help/help/t_SSL_VPN.html

Examples of newly-authenticated sessions from Cisco servers:

• openconnect#43 (comment 177677716): Session-Timeout and Session-Timeout-Remaining have the same value.

• https://www.mail-archive.com/openconnect-devel@lists.infradead.org/msg00968.html: Session-Timeout and Session-Timeout-Remaining are none, however Lease-Duration is set, with the maximum value.

  I don't understand why Session-Timeout is sometimes unset, while Lease-Duration is set. It's not necessary to reproduce this inconsistency in ocserv, though the OpenConnect client should interpret Lease-Duration as a fallback if Session-Timeout is none or missing. (See openconnect!156 (merged).)

My own testing of reconnected sessions (on a newer Cisco server supporting DTLS 1.2) shows that Session-Timeout-Remaining will have a value less than Session-Timeout, such that the expiration timestamp remains constant from one reconnection to the next.