BUG: RollingPanel was not behaving correctly in corner cases.

There quite some bugs in certain corner cases. Dropping of obsolete
axes was not working correctly, roll over could cause obsolete axes
to not drop. The tests are much more stringent now as well.

Also speeds up prior behavior up 10x due to concatenating Panels
instead of .update()ing them.
STY: Move RollingPanel to old .update() method to remain backwards compat with pandas==0.12.0.
PERF: Use concat() instead of .update().

Under the hood, .update() loops elementwise through the whole 3D array
which is obviously very inefficient. Instead, we can just concat() the
arrays we need which uses more efficient block-wise memory copying.
TST: Add out-of-order columns.