use a waitgroup to wait for reserve holders of MFiles before unmapping #4806

Job	Run time
Format check	12s
Lint check	1m 2s
Lint OpenAPI	17s
Lint javascript	15s
Spec	1m 10s
Spec FreeBSD	2m 39s
Filters for frontend specs	4s
Compile LavinMQ	1m 2s
Spec OS X	1m 38s
Test in release mode	18s
RabbitMQ HTTP API Go client test	59s
RabbitMQ HTTP API Ruby client test	48s
Bunny client test	2m 35s
RabbitMQ java client test	4m 59s
Frontend specs	0s
	17m 58s

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