Make the emit and event names once instead of on every event

[nigrosimone]

Connection::Emit() creates a new string for the event name and calls runInAsyncScope(handle(), "emit", ...) on every readable or writable event. That overload creates the "emit" string again and looks it up by a name that is not internalized, which costs more than the rest of the call. This makes the three names once per connection, internalized. emit is still looked up on every event, so replacing it keeps working.

A microbenchmark of the same call (a native object with EventEmitter.prototype two levels up, one listener, Node 26): 662 to 366 ns per event. With pg-native, sequential SELECT 1 on a local Postgres, 20 rounds: client CPU per query 56.96 to 55.99 us (-1.8%; the same build against itself 0.997).

The tests pass (84).