Email/Query calculate total

[remk]

###WHY
As a client i want to be able to calculate the total of mail returned by the query (ignoring pagination)

calculateTotal: Boolean (default: false) Does the client wish to know the total number of results in the query? This may be slow and expensive for servers to calculate, particularly with complex filters, so clients should take care to only request the total when needed.

request :

[[ "Email/query",{
  "accountId": "ue150411c",
  "calculateTotal": true,
  "filter":  {
     "minSize":  512
}
}, "0" ]]

response :

[[ "Email/query", {
  "accountId": "ue150411c",
  "total": 115,
  "ids": [ "Ma783e5cdf5f2deffbc97930a",
    "M9bd17497e2a99cb345fc1d0a", ... ]
}, "0" ]]

HOW

• accept a 'calculateTotal' parameter in the request. If true calculate the total number of results for the query.
• add a 'total' field in the response indicating the total number of results for the query.

DOD

• write an integration test validating the calculation of the total and the response to the client for both a simple and a complex query

[chibenwa]

What are the Mailbox level API changes required to support "calculate total" ? What is the implementation strategy with ElasticSearch?