Prepare version 2.12.0 release.

[smithsz]

2.12.0 (2019-03-28)

• [NEW] Added partitioned database support.
• [FIXED] Bug where document context manager performed remote save despite uncaught exceptions being raised inside with block.
• [FIXED] Fixed parameter type of selector in docstring.
• [FIXED] Removed internal Document._document_id property to allow a safe use of dict's methods.
• [IMPROVED] Performance of Result iteration by releasing result objects immediately after they are returned to the client.
• [IMPROVED] Updated Getting started section with a get_query_result example.
• [IMPROVED] Updated Result iteration by paginating with views' startkey and queries' bookmark.