Use ReadTier::kMemtableTier in read-before-write for max_successive_merges

Currently, `max_successive_merges` performs a read-before-write from disk to replace the current merge value in the memtable with a `put`. That read is blocking, which stalls the current thread. It would be better if `max_successive_merges` performed the read only within the memtable tier, replacing the current set of merge operands with the result of partially merging them. This likely requires introducing a new `ValueType`, and maybe a new toggle.