Move datastore code into a subpackage

[dhermes]

This PR was created via: https://gist.github.com/dhermes/e239691aa584bd56a5352e34aad27cf3

export PROJECT_DIR="${HOME}/google-cloud-python"
export READMES_DIR="${HOME}/i-wrote-some-readmes-for-2357"
cd ${PROJECT_DIR}
git worktree add -b make-datastore-subpackage ../hotfix official/master
python make_commits.py \
  --git-root "${PROJECT_DIR}/../hotfix" \
  --package datastore \
  --package-name "Google Cloud Datastore" \
  --readme "${READMES_DIR}/datastore/README.rst" \
  --extra-requires $'    \'grpcio >= 1.0.0\',\n' \
  --maybe-omit $'omit =\n    */_generated/*.py\n'

NOTE: Since only bigtable and datastore had to touch the run_pylint and tox.ini configs for ignoring generated code, the 3rd commit is just written the old-fashioned way.

[tseaver]

LGTM

[dhermes]

As with #2416, Pushed to resolve rebase conflict and re-ran script. Won't wait on Travis since a) that could be awhile and b) this will be painful.