To be used by support-core, we need to be able to enumerate all active
channels. We do this via WeakHashMap so that references get
automatically garbage collected.

Unclosed channel will remain in memory forever, which also helps us find
those leaks.

... and see how they can be interpreted to aid diagnostics

JLS (http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.7) states
that a write  to non-volatile long variable can go in 32bit batch, so
without it, read could retrieve a completely bogus value.

There's no risk of writer contention here because that is serialized by
the context in which it gets invoked.