V2: Remove addListItem and setMapEntry from ReflectMessage

[timostamm]

The type ReflectMessage includes methods for adding list items and map entries. For example:

reflectMsg.addListItem(listField, "Springfield");
reflectMsg.setMapEntry(mapField, "SPP", "Springfield Power Plant");

The purpose of the methods is to provide a fast path that does not require the creation of a ReflectList or ReflectMap object with ReflectMessage.get:

reflectMsg.get(listField).add("Springfield");
reflectMsg.get(mapField).set("SPP", "Springfield Power Plant");

But it turns out that we get nearly the same performance in practical use cases by caching ReflectList and ReflectMap objects:

- fromBinary perf-payload.bin x 7,037 ops/sec ±0.17% (101 runs sampled)
+ fromBinary perf-payload.bin x 6,924 ops/sec ±0.21% (98 runs sampled)

Depending on the use case, the performance regression is about 1,5%. Since the API is much cleaner without the fast path methods, this PR removes them.