Constant mz_rocksdb::defaults::DEFAULT_AUTO_SPILL_MEMORY_THRESHOLD

pub const DEFAULT_AUTO_SPILL_MEMORY_THRESHOLD: usize = _; // 89_478_484usize

The default for spilling from memory to rocksdb is 2 write buffers. Some initial tests found that 2 write buffers were the minimum memory usage of rocksdb when processing small amounts of data.

The calculation is based on the MEMTABLE_BUDGET, and is inverting the logic here: https://github.com/facebook/rocksdb/blob/bc0db33483d5e79b281ba3137ebf286b2d1efd8d/options/options.cc#L632-L637