Wait for completion of the upload queue in flush_frozen_layer (#8550)

Makes `flush_frozen_layer` add a barrier to the upload queue and makes
it wait for that barrier to be reached until it lets the flushing be
completed.

This gives us backpressure and ensures that writes can't build up in an
unbounded fashion.

Fixes #7317

compute_tools/src/compute.rs

@@ -400,7 +400,15 @@ impl ComputeNode {
     pub fn get_basebackup(&self, compute_state: &ComputeState, lsn: Lsn) -> Result<()> {
         let mut retry_period_ms = 500.0;
         let mut attempts = 0;
-        let max_attempts = 10;
+        const DEFAULT_ATTEMPTS: u16 = 10;
+        #[cfg(feature = "testing")]
+        let max_attempts = if let Ok(v) = env::var("NEON_COMPUTE_TESTING_BASEBACKUP_RETRIES") {
+            u16::from_str(&v).unwrap()
+        } else {
+            DEFAULT_ATTEMPTS
+        };
+        #[cfg(not(feature = "testing"))]
+        let max_attempts = DEFAULT_ATTEMPTS;
         loop {
             let result = self.try_get_basebackup(compute_state, lsn);
             match result {