[opt](multi-catalog) Optimize remote scan concurrency. (apache#51415)

Problem Summary:

[opt] (multi-catalog) Optimize remote scan concurrency.
1. Use `ScannerScheduler::get_remote_scan_thread_num()` to replace
`config::doris_scanner_thread_pool_thread_num` when calculate max
scanners in the external table case.
2. Remove `parallel_scan_max_scanners_count` calculation logic.

Author: kaka11chen

be/src/pipeline/exec/file_scan_operator.cpp

@@ -39,9 +39,9 @@ Status FileScanLocalState::_init_scanners(std::list<vectorized::VScannerSPtr>* s
 
     auto& p = _parent->cast<FileScanOperatorX>();
     // There's only one scan range for each backend in batch split mode. Each backend only starts up one ScanNode instance.
-    size_t shard_num = std::min<size_t>(
-            config::doris_scanner_thread_pool_thread_num / p.query_parallel_instance_num(),
-            _max_scanners);
+    size_t shard_num = std::min<size_t>(vectorized::ScannerScheduler::get_remote_scan_thread_num() /
+                                                p.query_parallel_instance_num(),
+                                        _max_scanners);
     shard_num = std::max(shard_num, (size_t)1);
     _kv_cache.reset(new vectorized::ShardedKVCache(shard_num));
     for (int i = 0; i < _max_scanners; ++i) {
@@ -65,9 +65,8 @@ void FileScanLocalState::set_scan_ranges(RuntimeState* state,
     auto& p = _parent->cast<FileScanOperatorX>();
 
     auto calc_max_scanners = [&](int parallel_instance_num) -> int {
-        int max_scanners = config::doris_scanner_thread_pool_thread_num / parallel_instance_num;
-        max_scanners =
-                std::max(std::max(max_scanners, state->parallel_scan_max_scanners_count()), 1);
+        int max_scanners =
+                vectorized::ScannerScheduler::get_remote_scan_thread_num() / parallel_instance_num;
         if (should_run_serial()) {
             max_scanners = 1;
         }

be/src/vec/exec/scan/scanner_scheduler.cpp

@@ -323,11 +323,12 @@ void ScannerScheduler::_scanner_scan(std::shared_ptr<ScannerContext> ctx,
 }
 
 int ScannerScheduler::get_remote_scan_thread_num() {
-    int remote_max_thread_num = config::doris_max_remote_scanner_thread_pool_thread_num != -1
-                                        ? config::doris_max_remote_scanner_thread_pool_thread_num
-                                        : std::max(512, CpuInfo::num_cores() * 10);
-    remote_max_thread_num =
-            std::max(remote_max_thread_num, config::doris_scanner_thread_pool_thread_num);
+    static int remote_max_thread_num = []() {
+        int num = config::doris_max_remote_scanner_thread_pool_thread_num != -1
+                          ? config::doris_max_remote_scanner_thread_pool_thread_num
+                          : std::max(512, CpuInfo::num_cores() * 10);
+        return std::max(num, config::doris_scanner_thread_pool_thread_num);
+    }();
     return remote_max_thread_num;
 }