Found the hanging backend

[Julian Satchell <j satchell eris qinetiq com>]

For a long time, each query resulted in an extra zombie backend thread.

I was playing round with backend-textchain.cs, and commented out the
call to ConvertHtmlToText (this was giving me enormous duplicate clues).
The hanging stopped; I was getting back to 0 backends after each query.
I think using GtkHtml widget to the conversion is fragile; we only need
a much simpler parser - basically getting the text between the point
brackets.

I don't think creating a duplicate textblock is the right thing to do
anyway; it would be better to have a shared method or utility class that
textblock users can call to convert htmlblocks.

Julian