Bug 936402: Duplicate connection status in UnixSocketImpl, r=bent

UnixSocketImpl, which mostly runs on the I/O thread, doesn't control
its reference to UnixSocketConsumer. If the connection status is
stored in UnixSocketConsumer, the I/O thread can't read it safely.

This patch duplicates the connection status in UnixSocketImpl, where
reading from the I/O thread is safe. Methods of UnixSocketImpl don't
need to access mConsumer any longer to obtain the connection status.