Prefix the messaging.server module with an underscore

I'm assuming for now that we'll have a specific notifications
consumption API which will use this as an internal implementation
detail. We can make this public again in future if/when we know
what the use case for it is.

Change-Id: I1b58fa651a11da60f9ed4b0eb33f073ba452f3e6
Prefix the executors module with underscore

With the MessagingServer API now private, we don't actually need to
expose the concept of an executor.

We may find in future that we want to support an executor type which
we don't want to include in the library itself but, for now, let's
be conservative.

Change-Id: I2c2902441e8e16c8bc620dfc1fbcdb69f8fe6665