MB-19672: Fix intermittent failure of 'test producer stream request (partial)'

MB-19672: Fix intermittent failure of 'test producer stream request (partial)'

This test was making incorrect assumptions about how / when
checkpoints would be created - it wasn't taking the checkpoint period
(how often we forcefully create new checkpoints) into account in it's
assuptions of checkpoint layout. As such it would occasionally fail if
it took longer than expected to run.

Fix this by essentially disabling chk_period (setting it to an
arbitrarily large value). At the same time make the tests expectations
explicit and check them where possible.

Change-Id: I9944f19d41a0a33064c9c43c8673c7ef9c4a3ab9
Reviewed-on: http://review.couchbase.org/69233
Reviewed-by: Manu Dhundi <manu@couchbase.com>
Tested-by: buildbot <build@couchbase.com>