Retry the xapi connection instead of latching it broken forever

[olivierlambert]

Fixes #89

Auth sets masterConnectionBroken when a XenAPI call times out, and OpenSession() never attempts another connection once the flag is set — the flag is never reset anywhere. A long-lived xsconsole (e.g. the one on the physical console) that experiences a single 15-second xapi hang therefore never talks to xapi again and shows Pool master is unreachable. forever, while newly launched instances work fine. Easy to hit during an HA pool-master failover, which is how it was reported: https://xcp-ng.org/forum/post/106689

This keeps the flag but as a cooldown rather than a latch:

• after a socket.timeout, connection attempts are skipped for CONNECTION_RETRY_SECONDS (60s), so an unresponsive xapi doesn't block the UI for the socket timeout on every update cycle — the behaviour the latch presumably existed to prevent;
• once the cooldown expires the next OpenSession() call tries again;
• a successful login clears the flag;
• the transitions are logged (XenAPI connection timed out - retrying in 60 seconds at most / XenAPI connection recovered), where previously the process went silent with no trace.

The timeout handling is factored into Auth.ConnectionTimedOut(), also used by the test-mode TCPAuthenticate() path so the retry timestamp is always set alongside the flag.

Validation

On XCP-ng 8.3: kill -STOP $(pgrep -x xapi), press F5 twice on the console xsconsole, kill -CONT after 60s.

Before (current master): the console is stuck on Pool master is unreachable. forever; only restarting xsconsole recovers it.

After (this branch, from the host's syslog):

17:44:58 xsconsole: XenAPI connection timed out - retrying in 60 seconds at most
17:45:24 bugtest: xapi unfrozen
17:45:59 xsconsole: XenAPI connection recovered

…and the console repopulates with live data on the next refresh, no restart needed.

[last-genius]

There are several pylint CI failures:

XSConsoleAuth.py:207:20: R0204: Redefinition of self.error type from Exception to XenAPI.XenAPI.Failure (redefined-variable-type)
XSConsoleAuth.py:207:20: R0204: Redefinition of self.error type from Exception to XenAPI.XenAPI.Failure (redefined-variable-type)
XSConsoleAuth.py:196:0: I0021: Useless suppression of 'redefined-variable-type' (useless-suppression)