feat(database): support the optional startAt key

[cartant]

Checklist

• Issue number for this PR: Passing an optional parameter to startAt() when querying FirebaseListObservable #362
• Docs included?: yes
• Test units included?: yes
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

This PR introduces support for the optional key parameter that can be passed to the SDK's startAt method. According to the SDK documentation, the parameter is "allowed if ordering by child, value, or priority".

There was some discussion in the linked (and closed) issue regarding the key parameter only being supported and useful when ordering by priority, but the documentation suggests that it is also supported - by startAt - when ordering by child and value. Indeed, were it not to be supported, it would not be possible to reliably page through lists that are ordered by child when there are large numbers of items with the same child value. This is an issue that I currently have with an implementation of a FirebaseListObservable-based infinite list.

There is a plunk here that demonstrates the use of key with the SDK to page when ordering by child.

Code sample

The key parameter is really only useful when paging through results, so this code sample is contrived and not particularly sensible:

let list = angularFire.database.list('some/data', {
  query: {
    orderByChild: 'some-child',
    startAt: { value: 'some-value', key: 'some-key' },
    limitToFirst: 10
  }
});

[cartant]

PS. This PR has no breaking changes.