[3.0] Optional syncing

[bchess]

From @Eta0 in #127 (review)

We could add a synchronize=False mode (referring to syncing data to disk) for incremental writes that defers writing headers until a write occurs with synchronize=True or the file is finalized. For cases like uploading to a temporary file before uploading to S3, writing headers can always be deferred, and this could be better for performance whenever constant synchronization isn't needed since it can guarantee a single write for that entire section, and (potentially) allow less fiddling with encryption information in the header section from having to repeatedly edit data in it (like MACs).