Several fixes

CHANGE: add timeout for py(shell,system)Call calls where appropriate
NEW: ProxyManagerClient - allow to specify validity and caching time separately
CHANGE: Shifter - use gProxyManager in a way that allows proxy caching
CHANGE: Subprocess - finalize the Watchdog closing internal connections after a command execution
FIX: dirac-dms-add-file - allow LFN: prefix for lfn argument
FIX: ProxyDB - replace instead of delete+insert proxy in __storeVOMSProxy
NEW: dirac-wms-job-get-output - allow to retrieve output for a job group
FIX: TaskQueueDB - fixed selection SQL in __generateTQMatchSQL()
CHANGE: OptimizerExecutor - reduce diversity of MinorStatuses for failed executors