BUG/MINOR: server: accept server IDs above 2^31 and clarify error message

BUG/MINOR: server: accept server IDs above 2^31 and clarify error message

Due to the check of the stored value instead of the parsed one, it was not
permitted to use server IDs above 2^31 while they are perfectly possible.
Let's refine the parsing and also update the error message to indicate the
range. The doc was also refined to reflect the relation with hash-key.

This may be backported though it wouldn't have any effect on working
configs.