StreamQueue: only unmap unused segments #3322

Job	Run time
Format check	16s
Lint and build HTTP API documentation	27s
Lint javascript	14s
Compile LavinMQ	2m 0s
Lint check	1m 29s
Spec	1m 46s
Spec OS X	1m 46s
Bunny client test	2m 44s
Test in release mode	12s
RabbitMQ java client test	5m 26s
RabbitMQ HTTP API Go client test	1m 23s
RabbitMQ HTTP API Ruby client test	55s
	18m 38s

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