Optimise metalayer snapshots

[neilalexander]

This PR optimises the marshalling of metalayer snapshots with a few techniques:

• Simplifies the custom marshalers for a number of types that appear often in the snapshotted config, as these were using reflection unnecessarily
• Simplifies the custom unmarshallers for the same types, as these were string-building more than necessary
• Attempts to reduce the number of reallocations that might happen in metaSnapshot() by preallocating slices

Signed-off-by: Neil Twigg neil@nats.io

[derekcollison]

Looking good. I left a comment on whether we should remove client info from snapshots for streams and consumers. We don't need it at that point and probably should not store it.

How are performance results looking?

[derekcollison]

I wonder if we need client here for snapshots? Both for streams and consumers?

[neilalexander]

I think we do, because there are quite a few places that stream assignments check for the service account and other fields once rehydrated from the snapshot.

[derekcollison]

You are correct we need account, but not much else I believe.

[derekcollison]

Let me know if you want a review now for merge.

[neilalexander]

Think this is fine for review to merge for now, looking into the ClientInfo separately.

[derekcollison]

LGTM