CONNECTION_ID() was implemented as a constant Item, i.e. an instance of
Item_static_int_func class holding value computed at creation time.
Since Items are created on parsing, and trigger statements are parsed
on table open, the first connection to open a particular table would
effectively set its own CONNECTION_ID() inside trigger statements for
that table.

Re-implement CONNECTION_ID() as a class derived from Item_int_func, and
compute connection_id on every call to fix_fields().