wait for servers to sync
Don't emit non existent fields when joining

Points in the joined series should have either the columns of the left
side or the columns of the right side of the join. Before this patch
join relied on merge to order the points of the two series and then join
consecutive points together. The merge emitted the union of the columns
of the two series, which caused the joined series to always have the
union of two series. This combined with the fact that the point's values
weren't adjusted to have nulls for the missing columns caused panic when
any operation is done on the points, e.g. addition.

Fix #740, Fix #781