Remove OptimisticTransactionDB

[tillrohrmann]

We are going to use a forked version of the rocksdb library to extend the bindings to expose more RocksDB functionality. Eventually, we will also expose the WriteBatchWithIndex which would allow us to read our own writes. Hence, we no longer need to use the OptimisticTransactionDB mode from RocksDB (right now, we actually don't require reading our own writes).

[tillrohrmann]

Postponed until we have WriteBatchWithIndex since we are relying on reading our own writes when applying multiple consecutive state mutations in the EffectInterpreter::pop_from_inbox.

[tillrohrmann]

There is an open issue for adding support for WriteBatchWithIndex to the rust-rocksdb binding rust-rocksdb/rust-rocksdb#836.