Check the length of prover-id in starting of client

[noel2004]

In db we have specified <30 chars for the prover_id column but there is no other constraint among our code. So coordinator may update a line with a string larger than 30 chars and this action seems cause the db conn has ruined persistently with following error ouput unless we restart the coordinator:

ERROR 2021-05-13T05:38:14Z: prover_cluster::coordinator::controller: db query idle task: Io(Kind(ConnectionAborted))

We may at least check the length at the beginning of a client starting. Or check it in corrdinator when accepting poll_task request if client can not be fully trusted.