Split block data into dedicated RocksDB blob files

[teor2345]

Motivation

We can increase Zebra's speed by splitting RocksDB block and transaction data into separate blob files.

Specifications

The new BlobDB can be configured (on a per-column family basis if needed) simply by using the following options:
enable_blob_files: set it to true to enable key-value separation.
...

https://github.com/facebook/rocksdb/wiki/BlobDB#api

Rust API

The current version of the rocksdb crate does not have Rust bindings for the blob options:
https://docs.rs/rocksdb/0.17.0/rocksdb/struct.Options.html

So we would have to use the raw option API:
https://docs.rs/rocksdb/0.17.0/rocksdb/struct.DBWithThreadMode.html#method.set_options_cf

Related Work

We might need to do this as part of lightwalletd support, if the performance isn't good enough. See #3134.

[teor2345]

We only need to do this if performance is unacceptable.

This could be a cause of:

• Finalizing blocks appears to be slow when checkpointing large blocks #3438