check postgres sequences for validity on startup

[richvdh]

in the last two weeks, I've seen two people who have migrated their databases to a new postgres instance and forgotten to copy the sequence generators. At best, this causes confusing errors; at worst, it could lead to corruption.

To save people from shooting themselves in the foot, we could check at startup that there are no rows in the table with IDs above the next number in the sequence, and refuse to start if so.

[richvdh]

in the meantime, to manually check that your database dump has been correctly imported to your new database:

SELECT last_value FROM state_group_id_seq;

... that should return a value significantly larger than 1.

[erikjohnston]

#8402 fixes this accept for the guest user ID generation, as per: https://github.com/matrix-org/synapse/pull/8402/files#diff-50fc898a46f43529763a3c63716ba0b4R44-R49 .