[CWB] Corpus installation

[Klaus Rothenhäusler]

Hi,
I made a small fix for a performance problem in xml_index_is_unique(). 
In our case it reduced installation time for a pre-indexed corpus (ca. 
600m tokens, 1.7m texts) from several hours - resulting in various 
timeouts - to a few seconds. The "svn diff" output is attached.

Cheers,
Klaus
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