use a waitgroup to wait for reserve holders of MFiles before unmapping #4790

Job	Run time
Format check	13s
Lint javascript	19s
Lint check	1m 5s
Lint OpenAPI	20s
Spec	2m 1s
Compile LavinMQ	1m 1s
Spec FreeBSD	2m 39s
Filters for frontend specs	3s
Spec OS X	1m 47s
Bunny client test	2m 30s
Test in release mode	19s
RabbitMQ HTTP API Go client test	59s
RabbitMQ java client test	5m 8s
RabbitMQ HTTP API Ruby client test	51s
Frontend specs	0s
	19m 15s

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