Generate package-locks for demo serving (#1220)

This change implements a package lock generator. This allows modules in a demo to be loaded using versions based on the resolved package lock result.

The package lock is generated by copying the `package.json` file, placing it an isolated temporary directory and running `npm install` in a separate process. Copying the `package.json` file is required to support both `dependencies` and `devDependencies`, which is a common use case wrt demos. This is an expensive operation which depends on the transitive dependency size. As such, there are number of performance improvements built in:
 * **Persistent backing** - generated package locks are permanently stored in Firestore. This is required to ensure load consistency since this is persisted across instances & deployments. As a result, the package-lock generation performance penalty will only be seen on the _first request for that package version_. To prevent users from ever seeing this latency, requests can be made to generate the package lock before any user needs it.
 * **In-memory cache** - a per-instance least recently used cache is used to improve performance of package lock reads.
 * **Data structure** - package locks are not stored in the same format that they are generated. The initial representation is very large and instead they are converted to a flat object with only package names & versions. This implies that conflicting versions present in `package-lock.json` are not preserved. This results in an ~100x compression vs original `package-lock.json` files (eg 432KB package-lock.json is now 6KB).
 * **Compression** - before storing into the in-memory cache, gzip compression is used to reduce memory and increase the number of items that can be cached. (For the 432KB package-lock.json, this is now 2.2KB).

To use the package lock, the HTML/JS rewriter now inserts the root package string as a query parameter to subsequent requests. These requests can then simply use the same package-lock to resolve versions.

demo/README.md

@@ -1,3 +1,20 @@
 # Demo service
 This is a service which enables demos to be served on webcomponents.org. It
 utilizes the unpkg service with support for bare module specifiers.
+
+## Developing
+Build this Typescript project:
+
+```
+npm run build
+```
+
+Run the tests:
+```
+npm run test
+```
+
+## Deployment
+This project requires [Cloud Firestore in Native mode](https://cloud.google.com/datastore/docs/firestore-or-datastore#choosing_a_database) when running on Google Cloud Platform. Ensure the project is built.
+```
+gcloud app deploy demo.yaml --project <your-project-id>

demo/demo.yaml

@@ -1,2 +1,7 @@
 runtime: nodejs8
-service: demo
+service: default
+
+instance_class: F4_1G
+
+automatic_scaling:
+  min_instances: 1