Optimise LSM compaction for topic data

[purplefox]

LSM can be configured to be more efficient for topic data (append only).

• Make SSTable size larger in lower levels - e.g. double size of table (up to a limit) on successive levels. This will reduce number of PUTs to S3 in compaction, thus reducing costs. It also makes historic consumers require less GETs.
• Consider making L1 max size very large. I.e. effectively have just two levels: L0 and L1 in the LSM. This will reduce number of S3 writes as compaction would typically only be done once. It can still be efficient as even if L1 gets very large, overlap of L0 to L1 can be small as data is append only.