multi-threaded-cf feature produces incompatible API--does not adhere to docs.rust-lang.org guidelines

[l-monninger]

The multi-threaded-cf feature changes the signature for rocksdb::DB::cf_handle. This is discouraged by docs.rust-lang.org.

Without:

rust-rocksdb/src/db.rs

Line 2191 in 30ffe0a

pub fn cf_handle(&self, name: &str) -> Option<&ColumnFamily> {

With:

rust-rocksdb/src/db.rs

Line 2218 in 30ffe0a

pub fn cf_handle(&self, name: &str) -> Option<Arc<BoundColumnFamily>> {

I noticed this while try to integrate two third-party workspaces both using cf_handle, i.e., as a consequence of unforeseen feature unification.