Optimize query by reading partition key from payload

[jannyHou]

See parent story https://github.com/strongloop/loopback-connector-cloudant/issues/214

Acceptance Criteria

Model Order has userId as its partition key.

• [x] Support providing the partitionKey in query, like Order.find({where: {somefield: 'somevalue', userId: 'auser',}})

  • In Cloudant.prototype.find(), if partitionKey is detected in the where clause, then invoke db.partitionedFind('auser', queryWithPartitionKeyExcluded)
  • Please note the partitionKey from options should override the one from where clause.

[jannyHou]

closing as PR merged