util.sql: Add API to start an explicit write transaction This will be used in SQLite3 to BEGIN IMMEDIATE, which attempts to acquire write locks immediately instead of when the first write statement is started. Thus it would fail faster if the write lock is already taken. Normally that should not occur but e.g. prosodyctl could take it. Investigating whether something similar is sensible in PostgreSQL, MySQL is a future task.

--- a/util/sqlite3.lua	Sun Sep 28 00:55:41 2025 +0200
+++ b/util/sqlite3.lua	Mon Aug 19 00:06:45 2024 +0200
@@ -295,6 +295,7 @@
 	end
 	return ok, ret;
 end
+engine.write_transaction = engine.transaction; -- TODO BEGIN IMMEDATE
 function engine:_create_index(index)
 	local sql = "CREATE INDEX IF NOT EXISTS \""..index.name.."\" ON \""..index.table.."\" (";
 	for i=1,#index do