use a waitgroup to wait for reserve holders of MFiles before unmapping #4795

Job	Run time
Compile LavinMQ	59s
Format check	13s
Spec FreeBSD	2m 22s
Lint check	1m 5s
Lint javascript	25s
Lint OpenAPI	17s
Filters for frontend specs	4s
Spec	1m 34s
Spec OS X	2m 17s
RabbitMQ HTTP API Ruby client test	56s
RabbitMQ java client test	5m 8s
Bunny client test	2m 39s
RabbitMQ HTTP API Go client test	1m 4s
Test in release mode	17s
Frontend specs	0s
	19m 20s

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