Lock sessions table when opening a new session

[sde1000]

If two people try to start a session at exactly the same time, two open sessions can be created.

Take an exclusive lock on the sessions table during session creation to prevent this.

[sde1000]

(Amazingly, this came up for the first time in 20 years, this week!)

[sde1000]

The easiest way to do this Properly™ would be to add a unique constraint on endtime with the SQL NULLS NOT DISTINCT clause — that way, at most one row could have a null endtime.

Unfortunately, this was only implemented in Postgresql 15 — most current installations are using Postgresql 14 (or earlier).

Since this has only come up once in 20 years, we could wait a while before fixing it...