Differentiate pool_unusable from pool_unworkable.
Use pool_unworkable in select_balanced as well.
Perform pool_resus on all pools that are found alive with the test pool threads.
Remove unused pthread_t typedefs from struct pool.
Set pool died on failed testing to allow idle flag and time to be set.
Close any open sockets when reusing a curl handle and reopen the socket whenever we're retrying stratum.