Join multiline postgres logs in compute_ctl (#5903)

Postgres can write multiline logs, and they are difficult to handle
after they are mixed with other logs. This PR combines multiline logs
from postgres into a single line, where previous line breaks are
replaced with unicode zero-width spaces. Then postgres logs are written
to stderr with `PG:` prefix.

It makes it easy to distinguish postgres logs from all other compute
logs with a simple grep, e.g. `|= "PG:"`

compute_tools/src/bin/compute_ctl.rs

@@ -350,7 +350,7 @@ fn main() -> Result<()> {
 
     // Wait for the child Postgres process forever. In this state Ctrl+C will
     // propagate to Postgres and it will be shut down as well.
-    if let Some(mut pg) = pg {
+    if let Some((mut pg, logs_handle)) = pg {
         // Startup is finished, exit the startup tracing span
         drop(startup_context_guard);
 
@@ -358,6 +358,12 @@ fn main() -> Result<()> {
             .wait()
             .expect("failed to start waiting on Postgres process");
         PG_PID.store(0, Ordering::SeqCst);
+
+        // Process has exited, so we can join the logs thread.
+        let _ = logs_handle
+            .join()
+            .map_err(|e| tracing::error!("log thread panicked: {:?}", e));
+
         info!("Postgres exited with code {}, shutting down", ecode);
         exit_code = ecode.code()
     }