chimera: Avoid integrity constraint violations on registering locations

When running 'pnfs register' on a pool to ensure that all files are registered,
PostgreSQL logs a warning about every file that was already registered:

2014-03-08 14:36:30 CET DETAIL:  Key (ipnfsid, itype, ilocation)=(000096B89DDF7E7E432A8CB4ED0F28543F69, 1, pool_1) already exists.
2014-03-08 14:36:30 CET STATEMENT:  INSERT INTO t_locationinfo VALUES($1,$2,$3,$4,$5,$6,$7)

More over, the transaction rollbacks appear to generate quite a lot of churn
between our master and slave instances.

This patch using a trick we already use when inserting new names in t_dirs:
Only insert when there is no existing record with the same key.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6699/