Add binary exponential backoff to _flush_writes

[blakehawkins]

This PR replaces an old PR that had rotted - I've not tested that this works as I don't have a local project running SheetSync.

Flushing changes to a remote document has some
probability of error for each batch. For large
changesets it becomes unlikely for writes to execute
successfully.

This commit causes writes to retry on failure using
exponential backoff, waiting 1, 2, 4, ... seconds
between writes, until success.