This class loads the mapper's SolrInputDocuments into one EmbeddedSolrServer
per reducer. Each such reducer and Solr server can be seen as a (micro)
shard. The Solr servers store their data in HDFS.
More specifically, this class consumes a list of <docId, SolrInputDocument>
pairs, sorted by docId, and sends them to an embedded Solr server to generate
a Solr index shard from the documents.