Refinery spends unnecessary 5% cpu resizing maps

[lizthegrey]

EnqueueEvent winds up looping over AddField() for a total of len(ev.Data) times.

Unfortunately due to behaviour in Go runtime (see golang/go#56182) we can't resize the map once we have it (nor do we have direct access to the map from libhoney clients since it's a private and lock-protected struct field).

Thus, we need to be able to pass in len(ev.Data) to builder.NewEvent() to allow correctly pre-sizing the map where the number of fields is known, and avoiding reallocating and copying the map over and over again for 5% of time spent in all of refinery.