Metadata records with hundreds+ of local xlinks take far too long to process

[simonp]

Large metadata records may have hundreds+ local xlinks (eg. <grg:itemClass xlink:href="#Item_Class"/>) many of which are links to the same element. searchLocalXLink is a bit dim - it just looks them up even if it has already done that thousands of times already. This is too slow - large documents with many local xlinks take more than two minutes to resolve. Fix is to keep a record of those we've already looked up and use that before we try a new lookup.

[simonp]

Fix is in svn commit rev 8900