See b/123287260. Currently backend.proto and distribution.proto live in
both googleapis/googleapis and googleapis/api-common-protos. The less
up-to-date one is being used (api-common-protos). This commit is created
by pulling down api-common-protos, deleting backend.proto and
distribution.proto, and running the generation scripts with COMMONPROTOS
set to the path of the modified api-commons-proto.

Notable changes include:

- Spanner BatchDML
- New client googleapis/ads/googleads/v1/
- New client googleapis/cloud/asset/v1/
- New client googleapis/cloud/scheduler/v1/

BREAKING_CHANGE_ACCEPTABLE=add ExecuteBatchDml method to interface

There are two definitions for distribution.proto: one in googleapis/googleapis,
and another in googleapis/api-common-protos. The canonical source is supposed
to be in the latter but is actually in the former. I've manually deleted
the distribution.proto in api-commons-proto on my local system and re-run
regen.sh to force the update in this PR.

This does a regeneration using the version of protobuf specified in the
go.mod: v1.2.0.

* regenerate

* using latest protoc-gen-go

* disable go1.6

gRPC imports golang.org/x/sys/unix which currently does not build on
go1.6. Depending on how compatibility story plays out, we'll either drop
1.6 support or add back 1.6 CI.

This commit also slightly changes the CI version "structure".
We previously list all Go versions to test against. This commit
makes us test against 1.7 and the latest version.
This means we don't have to manually bump version when new releases come
out. We have to be doing something really strange if we build against
1.7 and 1.10, but not 1.8 and 1.9.

Some .proto files are being moved from googleapis to api-common-proto.
However, one file will exist in both places for a while.

regen.go is modified to deduplicate files by name,
and regen.sh is modified to use api-common-protos in addition to
googleapis.

This update uses the latest version of the protoc Go plugin, which
reverted a change in UTF-8 validation for proto2 (and added fields
for proto3). This change removed validation of UTF-8 in proto2
and added a new field in proto3 that signals validation.

Relevant PR: https://github.com/golang/protobuf/pull/628

Matches commit https://github.com/googleapis/googleapis/commit/11639cabe4f5a3c93fc07b9d6ca3e144cd01f623.

Includes comment changes from updated protoc-gen-go.

* reimplement regen

* regenerate files

This change should fix our ongoing problem with duplicate file names causing failures.

At the moment, this changes requires you to build protobuf from source with this patch to work. Otherwise, we won't be able to properly import descriptor.proto.

- all proto files are deleted, as the new logic does not need to rewrite any part of the proto files.
- google.protobuf.descriptor is removed; it's official home is github.com/golang/protobuf/protoc-gen-go/descriptor.
- the protobuf directory is split into multiple packages. This is a breaking change.

Fixes GoogleCloudPlatform/google-cloud-go#419.

sed's -i flag differs on platform. Switch based on platform.

(Also moved api/distribution which had ended up in the wrong spot).

Fixes #5

Add back the appengine API.

The proto files within are dervied from:
  https://github.com/google/protobuf
  https://github.com/googleapis/googlapis