feat(sqlite3): add support for nested transactions using SAVEPOINT

It's possible to provide nested transaction semantics when using SQLite3
by using SAVEPOINT statements to manage transactions deeper than 1 level.

The implementation starts a transaction when transBegin() is first called as before.
But then all nested transactions will be managed by creating,
releasing (commiting) or rolling back savepoint.
Only when the outermost transaction is completed a COMMIT or ROLLBACK
will be executed, either committing the transaction and all inner
_committed_ transactions, or rolling everything back.

- SQLite Transaction: https://sqlite.org/lang_transaction.html
- SQLite Savepoints: https://sqlite.org/lang_savepoint.html

system/Database/SQLite3/Connection.php

@@ -55,6 +55,13 @@ class Connection extends BaseConnection
      */
     protected $busyTimeout;
 
+    /**
+     * How many savepoints have been created as part of managing nested transactions.
+     *
+     * @var int 0 = no savepoints, 1 = top level transaction + 0 savepoints, 2 = top level transaction + 1 savepoint, etc.
+     */
+    protected $savepointLevel = 0;
+
     public function initialize()
     {
         parent::initialize();
@@ -418,28 +425,76 @@ public function insertID(): int
         return $this->connID->lastInsertRowID();
     }
 
+    /**
+     * Generates the SQL for managing savepoints, which
+     * are used to support nested transactions with SQLite3
+     */
+    private function _savepoint($savepoint, string $action = ''): string {
+        return ($action === '' ? '' : $action . ' ') . 'SAVEPOINT `__ci4_' . $savepoint . '`';
+    }
+
+
     /**
      * Begin Transaction
      */
     protected function _transBegin(): bool
     {
-        return $this->connID->exec('BEGIN TRANSACTION');
+        $created = false;
+        $savepoint = $this->savepointLevel;
+        try {
+
+            return $created = ($this->savepointLevel === 0
+                ? $this->connID->exec('BEGIN TRANSACTION')
+                : $this->connID->exec($this->_savepoint($savepoint + 1)));
+        } finally {
+            if ($created) {
+                $this->savepointLevel++;
+            }
+        }
     }
 
     /**
      * Commit Transaction
      */
     protected function _transCommit(): bool
     {
-        return $this->connID->exec('END TRANSACTION');
+        if ($this->savepointLevel === 0) {
+            return false;
+        }
+
+        $committed = false;
+        $savepoint = $this->savepointLevel;
+        try {
+            return $committed = ($this->savepointLevel <= 1
+                ? $this->connID->exec('END TRANSACTION')
+                : $this->connID->exec($this->_savepoint($savepoint, 'RELEASE')));
+        } finally {
+            if ($committed) {
+                $this->savepointLevel = max($this->savepointLevel - 1, 0);
+            }
+
+        }
     }
 
     /**
      * Rollback Transaction
      */
     protected function _transRollback(): bool
     {
-        return $this->connID->exec('ROLLBACK');
+        if ($this->savepointLevel === 0) {
+            return false;
+        }
+        $rolledBack = false;
+        $savepoint = $this->savepointLevel;
+        try {
+            return $rolledBack = ($this->savepointLevel <= 1
+                ? $this->connID->exec('ROLLBACK')
+                : $this->connID->exec($this->_savepoint($savepoint, 'ROLLBACK TO')));
+        } finally {
+            if ($rolledBack) {
+                $this->savepointLevel = max($this->savepointLevel - 1, 0);
+            }
+        }
     }
 
     /**