With this patch, DynamoStore will now hold off writing anything to
the backend until it's got a full batch or someone calls UpdateRoot().
When it's time to write, DynamoStore will now fire off a new goroutine
to build the request (including compressing chunks), send it, and wait
for a response. It will keep up to dynamoWriteConcurrency concurrent
batch writes in flight.
Includes statKeeper, which provides a way for concurrent goroutines to
keep stats like write count, bytes written, etc. Also includes a refactor
to make unwrittenPutCache a separate type that both HTTPStore and
DynamoStore use instead of using copypasted code.
