Add "oldcpu" feature to switch to rocksdb v0.12

The v0.13 release does not work well with some old CPU models, see:

romanz/electrs#193
rust-rocksdb/rust-rocksdb#327
rust-rocksdb/rust-rocksdb#363

Cargo.toml

@@ -12,8 +12,9 @@ readme = "README.md"
 edition = "2018"
 
 [features]
-default = []
+default = [ "rocksdb" ]
 liquid = ["elements", "lazy_static"]
+oldcpu = [ "rocksdb-oldcpu" ]
 
 [dependencies]
 arraydeque = "0.4"
@@ -39,7 +40,8 @@ num_cpus = "1.12.0"
 page_size = "0.4.2"
 prometheus = "0.7"
 rayon = "1.3.0"
-rocksdb = "0.13.0"
+rocksdb = { version = "0.13.0", optional = true }
+rocksdb-oldcpu = { version = "0.12.4", optional = true, package = "rocksdb" }
 rust-crypto = "0.2"
 serde = "1.0.104"
 serde_derive = "1.0.104"

src/lib.rs

@@ -17,14 +17,19 @@ extern crate num_cpus;
 extern crate page_size;
 extern crate prometheus;
 extern crate rayon;
-extern crate rocksdb;
 extern crate serde;
 extern crate stderrlog;
 extern crate sysconf;
 extern crate time;
 extern crate tiny_http;
 extern crate url;
 
+// See https://github.com/romanz/electrs/issues/193 & https://github.com/rust-rocksdb/rust-rocksdb/issues/327
+#[cfg(not(feature = "oldcpu"))]
+extern crate rocksdb;
+#[cfg(feature = "oldcpu")]
+extern crate rocksdb_oldcpu as rocksdb;
+
 #[macro_use]
 extern crate chan;
 #[macro_use]