Initial ack persistance.
Add a test for setting only the family.
We don't actually need a row key binding for acks.
Extend test to verify contents of Put.
Filter acked rows from Scan.

This allows us to resume a partially succeeded import topology.

Unfortunately, we can't unit test this change because the filtering
happens server side, so we can't mock the behavior without delving
into the internals of HBase.
whitespace
The ack column is required in scan spout, change the default to null.

I think this will minimize surprise in the long run.  If/when we get around to a schema tool, we can bake in the default "_ack" name back in.

The other reason I'm not completely comfortable with the default column is it assumes a separate column family: I'm not actually sure what's best from a performance/operations point of view.
tweak
Include ack column in scans when configured.
Hoist common bits to simplify some tests.
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.