merge operator not associative

[yangl1996]

https://github.com/facebook/rocksdb/wiki/Merge-Operator#associativity-vs-non-associativity

Merge operators in blockchain module are not associative. e.g. https://github.com/yangl1996/prism-rust/blob/master/src/blockchain/mod.rs#L1610

Sometimes RocksDb tries to execute the merge operator in the backend, supplying it with values already in the database. This is invalid to our merge operators. E.g. in the case above, the operator takes a singe tuple, while the value stored in the table is a vector of another type.

[wgr523]

I tried to change merge operator. Please review this commit (might be private?) https://github.com/bagavi/prism-smartcontracts/commit/9d830d95ce2442d143c47dd9c1eac020d09f5df6

[yangl1996]

Will be fixed in the upcoming PR that gets rid of RockdDB in the blockchain db.