Fix "Concurrent modification during iteration" errors.

Author: rkistner

lib/src/connection_pool.dart

@@ -183,7 +183,12 @@ class SqliteConnectionPool with SqliteQueries implements SqliteConnection {
   @override
   Future<void> close() async {
     closed = true;
-    for (var connection in _readConnections) {
+
+    // It is possible that `readLock()` removes connections from the pool while we're
+    // closing connections, but not possible for new connections to be added.
+    // Create a copy of the list, to avoid this triggering "Concurrent modification during iteration"
+    final toClose = _readConnections.sublist(0);
+    for (var connection in toClose) {
       await connection.close();
     }
     // Closing the write connection cleans up the journal files (-shm and -wal files).

test/basic_test.dart

@@ -383,7 +383,6 @@ void main() {
         // Second connection to sleep more than first connection
         'SELECT test_sleep(test_connection_number() * 10)'
       ]));
-      await createTables(db);
 
       final future1 = db.get('SELECT test_sleep(10) as sleep');
       final future2 = db.get('SELECT test_sleep(10) as sleep');