Added support for character set conversion of query terms. The XML

configuration element query-charset specifies character set encoding
for target. In SRW/SRU mode, the proxy will convert from UTF-8 to
this encoding. Next, logical, step will be to honor Z39.50 character
set negotiation as well, so that Z39.50 queries will be converted
as well (we do not know encoding for Z39.50 sessions unless they say so).