use a waitgroup to wait for reserve holders of MFiles before unmapping #4794

Job	Run time
Lint javascript	17s
Format check	16s
Compile LavinMQ	1m 26s
Spec FreeBSD	1m 30s
Lint check	1m 6s
Lint OpenAPI	37s
Spec	1m 32s
Filters for frontend specs	2s
Spec OS X	1m 42s
RabbitMQ HTTP API Ruby client test	1s
RabbitMQ java client test	1s
Test in release mode	1s
Bunny client test	1s
RabbitMQ HTTP API Go client test	1s
Frontend specs	0s
	8m 33s

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