Rethink SQLSTATE code for ERRCODE_IDLE_SESSION_TIMEOUT.

 Rethink SQLSTATE code for ERRCODE_IDLE_SESSION_TIMEOUT.
 
 Move it to class 57 (Operator Intervention), which seems like a
 better choice given that from the client's standpoint it behaves
 a heck of a lot like, e.g., ERRCODE_ADMIN_SHUTDOWN.
 
 In a green field I'd put ERRCODE_IDLE_IN_TRANSACTION_SESSION_TIMEOUT
 here as well.  But that's been around for a few years, so it's
 probably too late to change its SQLSTATE code.
 
 Discussion: https://postgr.es/m/763A0689-F189-459E-946F-F0EC4458980B@hotmail.com