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.
Set sockd to false should curl setup fail on stratum.
Don't start testing any pools with the watchpool thread if any of the test threads are still active.