Make native debug flags toggleable at runtime

java-native-access#512)

… a memory adress that is directly accessible.

Motivation:

When we try to obtain a memory address of a ByteBuf we might need to
fallback to obain it via the ByteBuffer. When doing so we also need to
take the position of the ByteBuffer into account as it might be
non-zero.

Modifications:

- Add new static helper methods to Quiche class that can be used to
obtain the reader / writer memory address in a consistent / easy manner.
- Use these new methods

Result:

Always use the correct memory address and so don't end up with
data-corruption