We limit the number of concurrent blob downloads per node and per recovery, but if a user configures too many concurrent recoveries they may run out of per-node download permits which yields the following message:

> Unable to acquire permit to use snapshot files during recovery, this recovery will recover index files from the source node. Ensure snapshot files can be used during recovery by setting [indices.recovery.max_concurrent_snapshot_file_downloads] to be no greater than [25]

The `[25]` here is wrong, the 25 permits are shared across all inbound recoveries so we should divide the suggested value by `cluster.routing.allocation.node_concurrent_incoming_recoveries`. In extreme cases the user may have configured so many concurrent recoveries that the only reasonable limit is zero, and in that case we should suggest setting `indices.recovery.use_snapshots: false` instead.


server/src/test/java/org/elasticsearch/indices/recovery/RecoverySettingsTests.java
public void testGrantsSnapshotDownloadPermitsUpToMaxPermits() {
        ClusterSettings clusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
        RecoverySettings recoverySettings = new RecoverySettings(
            Settings.builder().put(INDICES_RECOVERY_MAX_CONCURRENT_SNAPSHOT_FILE_DOWNLOADS_PER_NODE.getKey(), 5).build(),
            clusterSettings
        );
        Releasable permit = recoverySettings.tryAcquireSnapshotDownloadPermits();
        assertThat(permit, is(notNullValue()));
        assertThat(recoverySettings.tryAcquireSnapshotDownloadPermits(), is(nullValue()));
        permit.close();
        assertThat(recoverySettings.tryAcquireSnapshotDownloadPermits(), is(notNullValue()));
    }
    public void testSnapshotDownloadPermitCanBeDynamicallyUpdated() {
        ClusterSettings clusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
        RecoverySettings recoverySettings = new RecoverySettings(
            Settings.builder().put(INDICES_RECOVERY_MAX_CONCURRENT_SNAPSHOT_FILE_DOWNLOADS_PER_NODE.getKey(), 5).build(),
            clusterSettings
        );
        Releasable permit = recoverySettings.tryAcquireSnapshotDownloadPermits();
        assertThat(permit, is(notNullValue()));
        assertThat(recoverySettings.tryAcquireSnapshotDownloadPermits(), is(nullValue()));
        clusterSettings.applySettings(
            Settings.builder().put(INDICES_RECOVERY_MAX_CONCURRENT_SNAPSHOT_FILE_DOWNLOADS_PER_NODE.getKey(), 10).build()
        );
        assertThat(recoverySettings.tryAcquireSnapshotDownloadPermits(), is(notNullValue()));
        assertThat(recoverySettings.tryAcquireSnapshotDownloadPermits(), is(nullValue()));
        permit.close();
    }
    public void testMaxConcurrentSnapshotFileDownloadsPerNodeIsValidated() {
        ClusterSettings clusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);