Add DEFAULT_MAX_BULK to regulate the maximum number of documents that may be sent at a time
in bulk through a DocManager. pysolr does not break large numbers of documents into separate
add requests, and pushing many documents through it at once may result in MemoryError.
fix: avoid StopIteration in bulk_upsert in solr_doc_manager.py
Use a separate collection to store namespace and timestamp metadata when using Mongo DocManager
Avoid string concatenation in order to support unicode namespace names in Python 2
Uniform exception handling across all DocManagers