Create Database Integration Test Suite

[gdbelvin]

Each database implementation has its own set of "unit tests"
We should really be testing these implementations at the interface layer so we can have some confidence that the assumptions we have about one storage implementation carry to the other implementations

[gdbelvin]

The storage integration test has revealed an interesting edge case.

Here's the cloud spanner documentation:

Commit might return an ABORTED error. This can occur at any time; commonly, the cause is conflicts with concurrent transactions. However, it can also happen for a variety of other reasons. If Commit returns ABORTED, the caller should re-attempt the transaction from the beginning, re-using the same session.

There are a new instances of

	err := admin.ReadWriteTransaction(ctx, func(ctx context.Context, tx AdminTX) error {

That do not retry on ABORTED.
These should probably be moved to RunInAdminSnapshot or RunInTransaction.
RunInX should also be modified to retry on ABORTED errors.

[gdbelvin]

@AlCutter for cloud spanner API comments

[gdbelvin]

Implementation ongoing:
#2134
#2133
#2132

[gdbelvin]

Working cloudspanner :-D
cc: @AlCutter @rmhrisk

[rmhrisk]

Woot woot ;)

[mhutchinson]

@pavelkalinnikov do you have any context on what work is remaining here?

[pav-kv]

integration/storagetest contains generic tests that can be used to test any storage implementation. If I remember correctly, it works for the storages in this repo, and what was left is making it work for our internal storage implementation. We can track it internally though.