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.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/contrib/dblink:
dblink.c (r1.60.2.7 -> r1.60.2.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.60.2.7&r2=1.60.2.8)