fixed handling `QueueMonitor.purgeLogsOlderThanDays` in Purge command #41

Job	Run time
Coding Standard & Static Analysis	26s
testsuite (8.1, sqlite)	17s
testsuite (8.1, mysql)	22s
testsuite (8.1, pgsql)	25s
testsuite (8.2, sqlite)	14s
testsuite (8.2, mysql)	35s
testsuite (8.2, pgsql)	29s
testsuite (8.3, sqlite)	15s
testsuite (8.3, mysql)	33s
testsuite (8.3, pgsql)	25s
	4m 1s

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