Lazy-load all GRPC dependencies

[schmidt-sebastian]

This PR is a fork of #196 and addresses a performance issue on Google Cloud Functions. Firestore usage on GCF is often read-only - that is, they provide the payload for a DocumentSnapshot and use Firestore only to deserialize this payload. We don't need to initialize the networking stack unless the user ends up writing back to Firestore.

This PR lazy-loads v1beta1 and @google-cloud/common.

The PR also removes @google-cloud/common-grpc (as this also pulls in the GRPC dependency). Unfortunately, this means that we no longer inherit from GrpcService. Is that a problem?

Test code:

const start = Date.now();
var Firestore = require('@google-cloud/firestore');
var snapshot = new Firestore().snapshot_('projects/foo/databases/bar/documents/coll/missing', {});
const end = Date.now();
console.log('Load time:', (end-start), 'ms');

Before: 473 ms, After: 34ms on a fully beefed-up Macbook

[codecov]

Codecov Report

Merging #197 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #197   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        1801   1800    -1     
=====================================
- Hits         1801   1800    -1

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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[alexander-fenster]

I'm not the one making the call here - @crwilcox is responsible for the libraries and @stephenplusplus and @callmehiphop support the common package.

[stephenplusplus]

The features of common-grpc/service & common-grpc/service-object are:

• Detect authentication
• Intercept all requests (including streams) to:
  • handle exponential backoff retries
  • allow auto-detection of project ID, replacing {{projectId}} placeholders
• Define common methods (delete, get, getMetadata, setMetadata)

If you're not using these things, then it should be safe to remove.

[schmidt-sebastian]

I confirmed that we are not using any of these features (by adding breakpoints), since all of our requests go through the Veneer layer. I have also tested a patched client that includes these changes and am able to write and read documents.