flat_mutation_reader: explicitly yield at every partition
flat_mutation_reader: explicitly yield at every partition

Right now we have yield points between partition processing guaranteed
by the fact that there are .get()s around the code, and those include
an yield point.

We have been discussing removing the implicit yield point from get and
pushing that to the caller. In that spirit, let's yield explicitly here
if needed.

It should be the responsibility of the loop that it doesn't hurt
latency, either by the fact that it is bounded by a small number of
iterations or yields. In other words, that loop should have a yield
point on every iteration (like the non-thread variant does).

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20180319173051.8918-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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