Re: [Patch] ALTER SYSTEM READ ONLY

The attached version is updated w.r.t. some of the review comments

from Soumyadeep and Andres.

Two thing from Andres' review comment are not addressed are:

1. Only superuser allowed to execute AlterSystemSetWALProhibitState(). As per
Andres instead we should do this in a GRANTable manner. I tried that but
got a little confused with the roles that we could use for ASRO and didn't see
any much appropriate one. pg_signal_backend could have been suited for ASRO
where we terminate some of the backends but a user granted this role is not
supposed to terminate the superuser backend. If we used that we need to check a
superuser backend and raise an error or warning. Other roles are
pg_write_server_files or pg_execute_server_program but I am not sure we should
use either of this, seems a bit confusing to me. Any suggestion or am I missing
something here?

2. About walprohibit.c/.h file, Andres' concern on file name is that WAL
related file names are started with xlog. I think renaming to xlog* will not be
the correct and will be more confusing since function/variable/macros inside
walprohibit.c/.h files contain the walprohibit keyword.  And another concern is due to
separate file we have to include it to many places but I think that will be
one time pain and worth it to keep code modularised.

Andres, Robert, do let me know your opinion on this if you think we should merge
walprohibit.c/.h file into xlog.c/.h, will do that in the next version.