mds: store session birth time on-disk in session map
PR https://github.com/ceph/ceph/pull/23314 adds session birth time to track average session age. During MDS failover (client switching to a new MDS), the birth time gets reset (as its tracked in-memory) thereby skewing the average session age perf counter and is somewhat incorrect when the session uptime is observer via "session ls" command.
For the fix, persist the session birth time in on-disk session map. On failover, use the stored session birth time rather than starting afresh.