Db, Multipartdb, Batch, and more; perf improv with __slots__

• add nlcodec-freqs CLI to setup.py
• log time and memory usage for learn task
• log BPE merge operations once every 2s instead of all operations
• using__slots__: ~25% faster, %30 less memory for BPE with 3M word types
• nlcodec.db.core with Db and MultipartDb
• nlcodec.db.batch with Batch and BathIterable
• CLI nlcodec.learn for learning BPE using pyspark
• CLI nlcodec.bitextdb to build a database from parallel text