backingstore: do not initialize memcache for debug builds

Summary:
Memcache is dogslow to initialize, taking >30s on debug build. As a
consequence, this slows down every single test by that amount time, with the
guarantee that no blobs will be found in memcache, ie: a total waste of time.

On release builds, Memcache is significantly faster to initialize, so let's
only disable initializing Memcache for debug builds only.

Reviewed By: fanzeyi

Differential Revision: D26800265

fbshipit-source-id: 8b393c603414de68268fdadb385de177e214a328

eden/scm/lib/backingstore/src/backingstore.rs

@@ -38,6 +38,9 @@ impl BackingStore {
             .local_path(&store_path)
             .suffix(Path::new("manifests"));
 
+        // Memcache takes 30s to initialize on debug builds slowing down tests significantly, let's
+        // not even try to initialize it then.
+        #[cfg(not(debug_assertions))]
         match MemcacheStore::new(&config, NullProgressFactory::arc()) {
             Ok(memcache) => {
                 // XXX: Add the memcachestore for the treestore.