[accumulo] A general "refresh" to the Accumulo binding (#947)

• Expand on the README, covering table creation and best-practices for table config
• Avoid unnecessary Text object creations (in loops and instead of byte[] usage)
• Use a ConcurrentHashMap to better match the DB API
• Fix error messages and always call printStackTrace() on exceptions
• Use BATCHED_OK instead of OK in insert() (more correct)