avro

Memory optimization in SchemaName.Fullname

FullName property is being called by `ClassCache.GetClass` for each property:
```csharp
public DotnetClass GetClass(RecordSchema schema)
{
	DotnetClass result;
	if (!this._nameClassMap.TryGetValue(schema.Fullname, ref result))
.
.
```

This allocates TONS of strings on a scenario which calls `Avro.Reflect.ReflectDefaultWriter.WriteRecord` on large amount of entities (millions) + large amount of fields (tens).