Chunk server: implement chunk directory checker io "timeout" logic by timing test file write / read, and successful / valid chunk file header loads. Do not declare chunk server "available" unless the io times is less or equal to the specified limit / "timeout". By default set directory checker timeout to 3/4 of disk io timeout value (3/4 * 4.5 minutes). Setting chunkServer.dirCheckerIoTimeoutSec to positive integer (number of seconds) can be used to override the default. Do not enforce chunk directory timeout on start up / initial chunk directory load. This logic, including timeout hysteresis, is intended to minimize chunk directory "oscillations" between available and not available states in the case of disk malfunction, and with the possible disk thrashing due to "external" to QFS disk "overload".