Add IndexedDB database implementation

[rafeca]

This PR adds an IndexedDB database implementation by building on top of https://github.com/atom/telemetry/pull/27.

This allows the telemetry package to keep events between sessions (so it fixes https://github.com/atom/telemetry/issues/26).

To interact with the IndexedDB API I've used the idb package, which is mostly a thin wrapper to expose the API using Promises.

Benefits of this library:

• It has no dependencies.
• It's super small (~1KB).
• Removes one of the biggest pains of dealing with the IndexedDB API (which is handling the IDBrequest objects and callbacks).
• Its API is still super similar to the original IndexedDB API, so it's not an abstraction layer that makes it hard to understand what's going on behind the scenes.
• It has high quality typescript type definitions (types for the stores and their items can be refined super precisely).

Next steps:

• This PR still does not solve the race condition issues. In order to fix them I can merge https://github.com/atom/telemetry/pull/25 which is still relevant.

[rafeca]

As suggested by @as-cii , I've started using electron-mocha to not have to mock indexedDB or localStorage. The change is in https://github.com/atom/telemetry/pull/28/commits/eadea5f94822b8b8d484e8c9ea79760c81929777