20559: Trim session count explicitly instead of using LRU cache.

20559: Trim session count explicitly instead of using LRU cache.

The LRU cache automatically implemented the session count limit by
dropping the oldest session, which would break our "one session per
token" rule (by evicting a session and creating a new one while the
old session was still in use) when there were more active sessions
than the configured limit.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>