Searching for prices during valuation no longer now properly excludes
price loops, avoiding near infinite looping with certain
configurations of market prices. Also we now always use a direct price
when available, rather than searching unnecessarily.
Price searching progress info, useful for troubleshooting, is now
displayed with --debug=2.
There could still be some corner cases we don't handle correctly. We
now give up with an error message if the searched price chains get too
long (> 1000). More importantly, we should also give up if the search
iterates too many times, but this is not done yet.