Function to handle after_update metadata tracking

[tantaman]

• Compress the clock table format #293
• lookaside for primary keys #344

Creating a lookaside table for primary keys ends up making triggers rather complex since they must either update or consult this lookaside in multiple places and we are not able to store variables in triggers.

In other words, we have a bunch of

INSERT OR IGNORE INTO lookaside (...);
SELECT (SELECT key FROM lookaside WHERE ...), ...

The update trigger also installs more than one trigger for update in order to handle primary key changes since changing a primary key is an update + delete.

Triggers do not run in a deterministic order in SQLite so each trigger would need to write the lookaside.

To try to simplify all this, I'm moving everything into a function so triggers will become:

CREATE TRIGER foo AFTER INSERT ON bar BEGIN
  SELECT crsql_after_update('table', new_primary_keys, old_primary_keys, new_columns, old_columns);
END;

and crsql_after_update will do all the correct thing.