first attempt at SolrJWriter thread pool
Merge branch 'master' into solrj_writer_thread_pool

Conflicts:
	lib/traject/solrj_writer.rb
Merge branch 'master' into solrj_writer_thread_pool
Phew, fix solrj_writer tests for batch/thread. Expected failure on raising exception under threaded at present
solrj_writer test, correct mock warning
solrj_writer test, do not commit on close, way too slow with real solr
solrj_writer, test commit_on_close only for mock
solrj_writer, crazy method of handling async exceptions with an async queue
Merge branch 'master' into solrj_writer_thread_pool
Merge branch 'master' into solrj_writer_thread_pool
SolrJWriter#skipped_record_count, thread-safely with AtomicInteger
SolrJWriter, better abstraction add_one_document with built-in rescue
SolrJWriter, move maybe_in_thread_pool outside of batch_add_documents, better more clear design
Indexer#process returns false on skipped records
command-line exits non-zero status code on Indexer#process returns false
move Traject::Indexer::Settings to it's own file, it's big enough now
fix OCLC extract
switch Writer#put to take a Context, not just a hash
fix oclc extract in demo_config again
SolrJWriter tracks the Context throughout, so can use it for error reporting (forthcoming)