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

- Use go mod tidy to check if deps were added without being added to go.mod.
- Use go mod download to download deps instead of go get.
- Use GO111MODULE=on everywhere applicable.
- Switch to being 1.12 aware.

Updates go.mod to include new version of gRPC, which removes its
dependency on the old github.com/golang/lint in favour of the new
golang.org/x/lint.

Relevant: https://github.com/google/go-genproto/pull/143

BREAKING_CHANGE_ACCEPTABLE=changes to RedisClient interface, which
should not be implemented by any user

apidiff currently fails our build when a breaking change is detected.
This needs tweaking, because sometimes we need to allow breaking
changes:

- An alpha|beta client gets a breaking change. This is acceptable
because breaking changes are expected to alpha|beta clients.
- A gRPC service interface gets a breaking change. This is acceptable
because users are not expected to implement gRPC service interfaces.
- A hard decision was made to allow a breaking change for some reason
like a bug fix.

This PR checks for the commit message to contain a
BREAKING_CHANGE_ACCEPTABLE=<reason>.

Includes new irm/ protos as well as updated speech protos.

It's too magical, and codeowners for this repo are likely to change
semi-frequently.

Mostly for cloud/talent/v4beta1

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 makes it easier to reason about what this script is doing.

Mostly for texttospeech, per internal bug 122452555.

While we're in the $GOPATH transitionary period, a dependency could
accidentally find its way into our require statements without being codified in
go.mod/go.sum. This CL makes our CI check for such a case, and exit 1 if it
detects it.

Mostly for the new firestore/v1 protos

This reverts commit 73d5040b.

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

This reverts commit 3360461a.

This commit was generated by go mod init && go mod tidy

This regeneration pulls in the latest generator
change that deprecates x/net/context in favour
of context. Since we no longer support Go
versions below 1.9, context is preferred.