lei_mail_sync: do not use transactions

lei_mail_sync: do not use transactions

For lei-index to work in parallel with MUA access and upcoming
inotify-based updates, mail_sync.sqlite3 needs to always be
up-to-date to read-only worker processes (ahead of everything
else).  So rely on the default auto-commit behavior and hope
SQLite WAL can reduce some of the overheads involved with
writes.