Support multithreaded migration

Currently, only one single thread is working on migration of data from solr to es. For huge data set, like millions of records in solr, this can consume a lot of time.

With this feature, users can specify an extra config `'num_workers'` to specify the number of parallel threads that will work on migration.
With multithreaded support, migration can happen a lot faster for huge data sets.

e.g.
```
curl -XPUT localhost:9200/_river/solr_river/_meta -d '
{
    "type" : "solr",
    "close_on_completion" : "true",
    "solr" : {
        "url" : "http://localhost:8383/solr/core-name",
        "q" : "*:*",
        "rows" : 5000
    },
    "index" : {
        "index" : "solr",
        "type" : "import",
        "bulk_size" : 5000,
        "max_concurrent_bulk" : 10,
        "num_workers" : 10
    }
}'
```