Make Zinc Logger reentrant/thread-safe

[rchristie]

When a client receives Zinc Logger events, other Zinc APIs may be in progress.
We don't expect the client to call other Zinc APIs from these events (reentrant code), however, we must ensure the Logger API itself is safe to call from its own events.
Best would be to ensure logger is thread-safe as well using proper synchronization objects.

[rchristie]

Ensure all notification APIs have warnings about making minimal zinc calls.
Check internal notification code to minimise risk.