Optional caching mechanism for table.lookup()

Inspired by work on git-history where I used this pattern:

    column_name_to_id = {}

    def column_id(column):
        if column not in column_name_to_id:
            id = db["columns"].lookup(
                {"namespace": namespace_id, "name": column},
                foreign_keys=(("namespace", "namespaces", "id"),),
            )
            column_name_to_id[column] = id
        return column_name_to_id[column]

If you're going to be doing a large number of table.lookup(...) calls and you know that no other script will be modifying the database at the same time you can presumably get a big speedup using a Python in-memory cache - maybe even a LRU one to avoid memory bloat.

The issue names table.lookup() and a Python cache pattern from git-history, but no implementation file or test. Start by locating table.lookup() and its existing tests, then establish the optional cache behavior, including the stated database-modification constraint and possible LRU approach. Done means repeated lookups can use an in-memory cache without unbounded memory growth.