turn count into countall

until we support NULLs, this is safe and efficient.

Fix #307.
myria: remove unused code

Myrialang should never compile a COUNT operator, at least not
until we have NULL support
myrial: test aggregates via logical plan also

Since we get rid of many aggregates before they make it to the physical plan
myrialang: re-run PushApply after distributed group by

DistributedGroupBy introduces Apply operators to finalize output
columns. Coalesce them.

Fix #306.
push apply into groupby

There may be further optimizations, but this catches some huge ones.

Fix #311, fix #298
Merge pull request #312 from uwescience/groupby-opts

groupby optimizations