added `QueueMonitor.enabled` option #26

Job	Run time
Coding Standard & Static Analysis	19s
testsuite (8.1, sqlite)	18s
testsuite (8.1, mysql)	23s
testsuite (8.1, pgsql)	24s
testsuite (8.2, sqlite)	13s
testsuite (8.2, mysql)	35s
testsuite (8.2, pgsql)	18s
testsuite (8.3, sqlite)	17s
testsuite (8.3, mysql)	24s
testsuite (8.3, pgsql)	17s
	3m 28s

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