Provide an option to increase per-level fanout with dynamic leveled to avoid extra levels

[mdcallag]

This is a feature request. More details on it are here.

In many cases a small increase in per-level fanout will avoid the use of an extra level as shown in the linked blog post above. My feature request is for RocksDB to support such a change with a cap on how much larger the per-level fanout can be. For example, I would accept a per-level fanout of 11 when the configured value is 10 if that allows the LSM tree to avoid creating an L2 that has a target size slightly larger than the target size for L1.