Optimize create, stat, remove operations

Currently, we check for the file's existence before going ahead and creating the file. This results in 2 RPCs (one to check and one to create) for a single create operation. Instead, we can have the same functionality with a single RPC by letting RocksDB's CreateOperand deal with that, i.e., a new entry can only be created if it does not exist. @tkaya's investigation confirmed that this is possible.

We already have a branch for this specific case but do not use it in this way. So, there is no overhead that is added.

This is similar to nextgenio io 500 experiment's create optimization where we removed the exist check from the path.

edit: renamed to Optimize create, stat, remove operations