...as otherwise the SvtMatchContext_Impl thread can continue to run for
arbitrarily long, and the other thread calling Stop() and join() will block.

However, especially the WebDAV UCP does not properly support aborting commands,
see 260afe56 " neon commands cannot be aborted",
so this is not yet enough to actually fix rhbz#915743 "thread deadlock/slow
join in insert->hyperlink in impress."

Change-Id: I0da899f824763e1b3d19bb5b38d906feb690b623