Add ability to write chunks without a length specified

[ecton]

One thing I did for speed is to require the length for each chunk, so that we can avoid needing to re-patch the length after the chunk is done writing.

However, for some workloads, it may be cheaper to do the extra IO than it would be to pre-compute the length. It shouldn't be too tricky to support this flow.

The documentation for this new function should describe the implementation so that users can make a judgement about whether they want to try to compute the chunk sizes or whether they want to let OkayWAL fill in the details for them.