feature: raft nodes networking

This patch allows two nodes to communicate over the net.box.

It consists of two major parts:

1. traft::ConnectionPool implements async communication on the client-side.
2. C stored procedure raft_interact handles server-side.

The third commit addresses parsing --peer command-line args.

Run it in two separate terminal windows:

PICODATA_RAFT_ID=1 cargo run -- run --listen 3301 --data-dir tmp/3301 --peer 127.0.0.1:3301,127.0.0.1:3302

PICODATA_RAFT_ID=2 cargo run -- run --listen 3302 --data-dir tmp/3302 --peer 127.0.0.1:3301,127.0.0.1:3302

Close https://gitlab.com/picodata/picodata/picodata/-/issues/17, https://gitlab.com/picodata/picodata/picodata/-/issues/18

picodata/main.rs

@@ -66,6 +66,8 @@ fn main_run(matches: &clap::ArgMatches) {
         envp.insert("PICODATA_PEER".into(), peer);
     }
 
+    envp.entry("PICODATA_PEER".into())
+        .or_insert_with(|| "127.0.0.1:3301".into());
     envp.entry("PICODATA_LISTEN".into())
         .or_insert_with(|| "3301".into());
     envp.entry("PICODATA_DATA_DIR".into())