move existing client back to current namespace

[daniel-sanche]

At the start of v3 development, I moved the existing client into google.cloud.bigtable.deprecated. For the alpha, we'll need it to be back in it's original namespace.

The new client should be moved to google.cloud.bigtable.data to avoid name collisions with duplicated model classes

Along with this change, we should make sure required public classes are suffixed with Async