This adds the core of the columnar serialization code paths. Even though
we internally scan in a columnar fashion in the tablet server, sending
those columns across the wire isn't straightforward. We have two bits of
necessary processing:

1) the selection vector needs to be taken into account so we only send
back selected rows. This means we need to copy out the selected cells
and also copy out the selected bits from the null bitmap where relevant.
Doing the null bitmap portion efficiently with wide platform support
makes up a lot of this patch.

2) for the case of null values, we want to make sure we don't send
uninitialized memory (which might include secrets!) to the client. So we
need to zero out any cells where the corresponding non-null bitmap bit
is unset.

To keep the review manageable, this just adds some unit tests and all
the new code is initially "dead". Later commits will add the parts that
construct the full block of columns to be sent on the wire, hook this
into the tserver, etc.

Change-Id: I16f2993081aac54609aab4d8219ef0bf6c7708c2
Reviewed-on: http://gerrit.cloudera.org:8080/15556


Tested-by: Kudu Jenkins
Reviewed-by: Andrew Wong <awong@cloudera.com>
Reviewed-by: Alexey Serbin <aserbin@cloudera.com>