Add a mutex to the sqlite3_db_config() interface so that it is threadsafe

when two or more threads call it on the same database connection at the same
time.

FossilOrigin-Name: 459ad8846ee1ee2d3b277a291c47121692bdf477e779b06e77be8338f62237a6

src/main.c

@@ -915,6 +915,7 @@ int sqlite3_db_cacheflush(sqlite3 *db){
 int sqlite3_db_config(sqlite3 *db, int op, ...){
   va_list ap;
   int rc;
+  sqlite3_mutex_enter(db->mutex);
   va_start(ap, op);
   switch( op ){
     case SQLITE_DBCONFIG_MAINDBNAME: {
@@ -980,6 +981,7 @@ int sqlite3_db_config(sqlite3 *db, int op, ...){
     }
   }
   va_end(ap);
+  sqlite3_mutex_leave(db->mutex);
   return rc;
 }