Support async stream serialization/deserialization

Our current implementation of DefaultSerializer doesn't support async serialization/deserialization. However, it is reasonable to assume that other custom serializers may support async. For example, Microsoft's new high-perf System.Text.Json library supports async.

IDataMapper and ITypeSerializer should offer async overloads for these use cases, which should preferably be added before the 3.0.0 release. DefaultSerializer can just implement them synchronously. The SearchDataMapper implementation can actually use the async approach now, as written.

The synchronous methods should be maintained, as they will be more performant for K/V operations (which are writing to MemoryStream instead of the network).