Fixed SegmentedQueues not being cleaned up on session purge

[hylkevds]

Segmented queues were closed, but not cleared on session close. This resulted in segments not being returned to the pool, and thus disk-space not being released. The queues were also not removed from the QueuePool, resulting in a memory leak.

Relates #734

[hylkevds]

Hi @hylkevds thanks for this PR. I would also add unit test on the unsafequeues.Queue or unsafequeues.QueuePool to verify that a queue purge or close give back the segments to be reused.

I'll look into making some tests for this.

[hylkevds]

@andsel: I added a test to verify the queue cleanup and page-file re-use.

[andsel]

Thanks @hylkevds for your contribution to the project!