pgsql: Rearrange dblink's dblink_build_sql_insert() and related routines

Log Message:
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Rearrange dblink's dblink_build_sql_insert() and related routines to open and
lock the target relation just once per SQL function call.  The original coding
obtained and released lock several times per call.  Aside from saving a
not-insignificant number of cycles, this eliminates possible race conditions
if someone tries to modify the relation's schema concurrently.  Also
centralize locking and permission-checking logic.

Problem noted while investigating a trouble report from Robert Voinea --- his
problem is still to be fixed, though.

Modified Files:
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    pgsql/contrib/dblink:
        dblink.c (r1.94 -> r1.95)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.94&r2=1.95)