feature: pass raft_id as an env var

Run it:

```
PICODATA_RAFT_ID=2 cargo run run
```

This is a pre-requisite for further two-node configuration development.
This is a temporary hack until fair joining is implemented.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/16

picolib/lib.rs

@@ -121,8 +121,41 @@ fn main_run() {
     );
 
     traft::Storage::init_schema();
+
+    let raft_id: u64 = {
+        // The id already stored in tarantool snashot
+        let snap_id: Option<u64> = traft::Storage::id();
+        // The id passed in env vars (or command line args)
+        let args_id: Option<u64> = std::env::var("PICODATA_RAFT_ID")
+            .and_then(|v| {
+                v.parse()
+                    .map_err(|e| panic!("Bad PICODATA_RAFT_ID value \"{}\": {}", v, e))
+            })
+            .ok();
+
+        match snap_id {
+            None => {
+                let id: u64 = args_id.unwrap_or(1);
+                traft::Storage::persist_id(id);
+                id
+            }
+            Some(snap_id) => match args_id {
+                Some(args_id) if args_id != snap_id => {
+                    panic!(
+                        "Already initialized with a different PICODATA_RAFT_ID:
+  snapshot: {s}
+ from args: {a}",
+                        s = snap_id,
+                        a = args_id
+                    )
+                }
+                _ => snap_id,
+            },
+        }
+    };
+
     let raft_cfg = raft::Config {
-        id: 1,
+        id: raft_id,
         applied: traft::Storage::applied().unwrap_or_default(),
         ..Default::default()
     };

picolib/traft/storage.rs

@@ -86,6 +86,10 @@ impl Storage {
             .unwrap()
     }
 
+    pub fn id() -> Option<u64> {
+        Storage::raft_state("id")
+    }
+
     pub fn term() -> Option<u64> {
         Storage::raft_state("term")
     }
@@ -118,6 +122,15 @@ impl Storage {
         Storage::persist_raft_state("vote", vote)
     }
 
+    pub fn persist_id(id: u64) {
+        // We use `insert` instead of `replace` here
+        // because `id` can never be changed.
+        Space::find(SPACE_RAFT_STATE)
+            .unwrap()
+            .insert(&("id", id))
+            .unwrap();
+    }
+
     pub fn entries(low: u64, high: u64) -> Vec<raft::Entry> {
         let mut ret: Vec<raft::Entry> = vec![];
         let space = Space::find(SPACE_RAFT_LOG).unwrap();