Fix starting a usage search while another one is running.

Theoretically it could now run multiple searches in parallel, but since
a single search grabs all available threads from the pool, any search
request afterwards is queued for execution when the previous one
finished.

Task-number: QTCREATORBUG-6101
Change-Id: Ifada0ccdd1ce7cfb4aaaaa4de2db070025f91a3a
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>