doc: add comment on method to set savepoint_limit dynamically savepoint_limit isn't a real config option as in config.ini. I generate one on the fly for use in the backend during import. While this is arguably a valid config option, it is also something that is niche (import a tracker to postgres) and adds clutter to config.ini.

try:
    # Python 3+
    from time import perf_counter
except (ImportError, AttributeError):
    # Python 2.5-2.7
    from time import clock as perf_counter   # noqa: F401