Allow LSM to merge into custom data sources

LSM asks very little of data sources used for merges: we do a bulk load, then non-transactional queries. Upper layers take care of caching and in-flight transactions, keeping things simple for the data source.

This makes it an attractive way to test integration of new data sources with WiredTiger.

Add support for configuring LSM merges into a custom data source (i.e., not "file:").