feature: Consider allowing opening the database in "read-only" or "secondary" mode

[upbqdn]

Motivation

Zebra currently uses the open_cf_descriptors function here to open the database.

I was using the zebra-scan crate in a separate binary to access the scanning results while running Zebra. The crate tried to open the same database as Zebra in read-write mode, and since Zebra was running, I couldn't access the database.

RocksDB supports opening the database in "read-only" or "secondary" mode, which would allow separate binaries that use, for example, the zebra_scan crate, to read the scanning results while Zebra is running.

Specifications

RocksDB's "read-only" and "secondary" modes: https://github.com/facebook/rocksdb/wiki/Read-only-and-Secondary-instances

Rust bindings:

• open_cf_descriptors_as_secondary
• open_cf_descriptors_read_only