Asynchronous writes to SDAPI

Experimental feature to enable fully asynchronous writes for SDAPI writes to the cloud. This replaces the existing "segsplit" feature in OpenZGY and sometimes gives better performance.

The code as of this writing has not been tested enough for general use.

Enable by setting OPENZGY_NUMTHREADS_WRITE to the desired number of threads. Typically that would be the same number that was configured for segsplit. Turning on the new feature will silently turn off segsplit.

For detailed documentation see doc/async-sdapi-write.md.

Also in this merge request, unrelated to the above:

Ensure unique names when publishing packages.