Fix race condition in io::Stats

The workerStats std::vector was modified concurrently without being
synchronized. The Stats are constructed together with IoContext and
the worker IoContexts are constructed concurrently at the beginning of
the worker loop. And in the Stats constructor the workerStats
std::vector was modified by adding the Stats instance that is
currently being constructed to it.

Turns out, we don't need the workerStats data structure at all. We
simply provide the global and worker IoContexts to
io::Stats::printStats(). This results IMHO in a cleaner design of
printStats() since there is no longer a data structure called
workerStats, that in fact, not only holds the worker's stats. And
while we at it, we rename io::Stats::printWorkerStats() to
io::Stats::printsStats(), since it does, in fact, not only print the
worker stats, but more or less all related worker stats.