added `QueueMonitor.enabled` option #27

Job	Run time
Coding Standard & Static Analysis	44s
testsuite (8.1, sqlite)	13s
testsuite (8.1, mysql)	22s
testsuite (8.1, pgsql)	34s
testsuite (8.2, sqlite)	14s
testsuite (8.2, mysql)	33s
testsuite (8.2, pgsql)	36s
testsuite (8.3, sqlite)	18s
testsuite (8.3, mysql)	22s
testsuite (8.3, pgsql)	20s
	4m 16s

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