Async test fix race condition, part IV

Export and pulse image attachment refactor

Previously images in pulse emails were serialized to bytes and the
bytes were kept in map stored in an atom. After all of the graphs were
serialized, they were written to disk and the file handle was passed
to the email library.

This commit immediately writes the file to disk and returns a
URL. This will no longer hold onto the memory need for all of the
cards and can be GC'd right away. This also avoids writing static
images like the no results image to disk evertime, rather than just
reading it out of the JAR.

This simply moves the functions from the API layer to a utility
namespace so that the pulse code can make use of the CSV/XLS export
logic.

Async test fix race condition, part III

Virtualize AccordianList to resolve performance issues with many dbs/schemas/tables/fields

Fix async test race conditions, part II

Add a noop `-init-driver` function to `metabase.driver.google`

Remove ^:always-validate metadata

Add more ordering to pulse_channel_test results

The data comes right out of the database without an order applied so
occasionally there are transient failures due to the order being
different. This just dumps the data in a set since the order isn't important.

A warning is currently shown as we assume that namespace is a
driver. It's used by driver code but is not itself a driver. Added a
noop function so it won't output that warning.

Code cleanup 

Fix potential race condition where the job gets cleaned up too soon.