feat: promisify findbyid

[jannyHou]

Signed-off-by: jannyHou juehou@ca.ibm.com

A follow up PR for https://github.com/strongloop/loopback-connector-cloudant/pull/241 See discussion from https://github.com/strongloop/loopback-connector-cloudant/pull/241#issuecomment-638860551 This PR:

• promisifies the connector level findById so that lb4 app can call it using await connector.findById()
• check if the found instance belong to the query model, if not throw error.

Checklist

• [ ] DCO (Developer Certificate of Origin) signed in all commits
• [ ] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [ ] Code conforms with the style guide
• [ ] Commit messages are following our guidelines

[jannyHou]

The cloudant failure should be related:

 2 failing
14:58:46 
14:58:46   1) cloudant count.test - imported from couchdb2
14:58:46        count
14:58:46          "before all" hook for "returns more than 25 results with global limit set":
14:58:46      Error: Timeout of 40000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (/home/jenkins/workspace/ds/loopback-connector-cloudant~master/test/imported.test.js)
14:58:46       at listOnTimeout (internal/timers.js:554:17)
14:58:46       at processTimers (internal/timers.js:497:7)
14:58:46 
14:58:46   2) cloudant create.test - imported from couchdb2
14:58:46        create
14:58:46          "before all" hook for "creates a model instance when `_rev` is provided":
14:58:46      Error: Timeout in connecting after 5000 ms
14:58:46       at Timeout._onTimeout (node_modules/loopback-datasource-juggler/lib/datasource.js:2654:10)
14:58:46       at listOnTimeout (internal/timers.js:554:17)
14:58:46       at processTimers (internal/timers.js:497:7)
14:58:46