pgsql: Force pg_database updates out to disk immediately after ALTER

Log Message:
-----------
Force pg_database updates out to disk immediately after ALTER DATABASE;
this is to avoid scenarios where incoming backends find no live copies
of a database's row because the only live copy is in an as-yet-unwritten
shared buffer, which they can't see.  Also, use FlushRelationBuffers()
for forcing out pg_database, instead of the much more expensive BufferSync().
There's no need to write out pages belonging to other relations.

Modified Files:
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    pgsql/src/backend/commands:
        dbcommands.c (r1.146 -> r1.147)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.146&r2=1.147)