The ChunkedInsert object is great for speeding up inserts. This commit
does the same for updates. It chunks up updates then sends them to the
table's `update_many` in bundles that contain updates to the same
fields.
In doing this the ChunkedInsert and new ChunkedUpdate were refactored to
inherit from a common `_Chunker` object that does most of the chunking
logic. This should also make it simple to add a ChunkedUpsert object.
Micha Gorelick
ff98cfc5d4