* object: fixed race condition between Result() and Checkpoint()
This would sometimes result in indirect objects having empty object IDs.
Fixes#648
* upload: ensure checkpoints never containt empty object IDs.
* testing: reduce armhf test weight
* performance: added buf.Pool which can be used to manage ephemeral buffers for encryption and compression
* repo: switched object writer to buf.Pool
* content: switched encryption to use buf.Pool
* object: switched compression to use buf.Pool
* testing: added missing content manager Close()