Added support for namespaces to RocksDb kvstore.

[web3-developer]

This PR contains the first part of the namespaces/rockdb column family changes.

Changes in this PR:

• Refactor/cleanup of the existing RocksStoreRef type.
• Added new RocksNamespaceRef type.
• Removed the unused and unneeded db backups feature from the RocksStoreRef and other locations.
• Removed the usage of the RocksDb readonly option from the RocksStoreRef as it complicates the API and was unused. Also the read only RocksDb instances have some issues to be aware of. Basically when starting a read only db connection the data read is from the point in time when the connection was opened but new updates won't be reflected in that connection. See here: https://github.com/facebook/rocksdb/wiki/Read-only-and-Secondary-instances
• Bumped nim-rocksdb to the latest version which contains a recent fix to always open the default column family even if it is missing from the column families list.