Rework the read-only handling

Instead of using the updater reference as the indicator of read-only
views, add an explicit boolean. Thus all instances reuse a single
updater reference, but still guard access to it via an explicit check.

This changes the behavior a bit: instead of throwing NPE on read-only
views, we throw IllegalStateException. The reason for that is that we
should have caught the attempt in the MAP API functions.

Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Merge pull request #15 from rovarga/outstanding

Rework read-only tracking