Support multiple rescores

Detects if rescores arrive as an array instead of a plain object.  If so
then parse each element of the array as a separate rescore to be executed
one after another.  It looks like this:
   "rescore" : [ {
      "window_size" : 100,
      "query" : {
         "rescore_query" : {
            "match" : {
               "field1" : {
                  "query" : "the quick brown",
                  "type" : "phrase",
                  "slop" : 2
               }
            }
         },
         "query_weight" : 0.7,
         "rescore_query_weight" : 1.2
      }
   }, {
      "window_size" : 10,
      "query" : {
         "score_mode": "multiply",
         "rescore_query" : {
            "function_score" : {
               "script_score": {
                  "script": "log10(doc['numeric'].value + 2)"
               }
            }
         }
      }
   } ]

Rescores as a single object are still supported.

Also add documentation on score_mode when adding documentation about multiple
rescores.

Closes #4748
Closes #4742