castorini/Anserini

(Re-)add object pooling for SolrClients... it helped throughput quite a bit for `CloudSolrClient` even
though it's thread-safe. Also, buffer docs for the `ConcurrentUpdateSolrClient` as it's queue is for 
update requests (i.e., it buffers `add(SolrInputDocument)` calls until the queue size is reached then 
drains the queue concurrently).