Support exporting of all stored fields for search results

[sv3k]

According to documentation, ES supports retrieving all stored fields in search results using "*", so this request should work:

curl -XPOST 'localhost:9200/test/test/_export' -d '{
   "query" : {
       "match_phrase" : {
           "key" : "value 1"
       }
   },
   "fields" : [ "*" ]
}'

But it doesn't, seems like because knapsack is using GET API which actually doesn't support this feature. We can probably switch to using POST API in order to support this, what do you think?