Batch websocket responses using the v3 protocol (#5061)

# Description of Changes

In keeping with the "pipeline everything" approach of SpacetimeDB, this
patch serializes multiple client updates in a single websocket message
using the v3 protocol.

# API and ABI breaking changes

None. `spacetime subscribe` was updated to use the v3 websocket api, but
it falls back to v1 if protocol negotiation fails.

# Expected complexity level and risk

2

# Testing

This patch updates `spacetime subscribe` to use the v3 websocket
protocol by default in order to get adequate coverage via the
smoketests.

Author: joshua-spacetime