whitespace
Start/stop flusher task based on member nullity.
move constant
extract method.
demote method access from public to package-private.
Extract flushAndAck(), keep flush() (which no longer acks).
Parameterize flusher, but don't actually add any new code branches.
Refine test to verify method called by flush task.
name some mocks
Push open calls out of fixture.

The pending{1,2} assignments need to come with or else things explode.
Clean up tests a bit.
Flush out remaining tests.
Do not eagerly close scan spouts on exhaustion, they need to put acks.
ScanSpouts set the emitted row timestamps on ack puts.