Move xlog_utils.rs to postgres_ffi module.

I had copy-pasted these functions to a few other places. Clean that up,
move them to a common module, and add some comments.

control_plane/src/local_env.rs

@@ -16,7 +16,7 @@ use anyhow::Result;
 use serde_derive::{Deserialize, Serialize};
 
 use pageserver::ZTimelineId;
-use walkeeper::xlog_utils;
+use postgres_ffi::xlog_utils;
 
 //
 // This data structure represents deserialized zenith config, which should be