RFR: notify: use "listener" instead of "consumer"/"updater" in names

[Erik Helin]

Hi all,

please review this refactoring of the notifier bot that uses the terminology "listener" instead of
"consumer"/"updater". I also refactored how listeners are added to the `NotifierBot` (which now implements the
`Emitter` interface).

Testing:
- [x] `make test` on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - notify: use "listener" instead of "consumer"/"updater" in names

Changes: https://git.openjdk.java.net/skara/pull/651/files
 Webrev: https://webrevs.openjdk.java.net/skara/651/webrev.00
  Stats: 622 lines in 19 files changed: 302 ins; 254 del; 66 mod
  Patch: https://git.openjdk.java.net/skara/pull/651.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/651/head:pull/651

PR: https://git.openjdk.java.net/skara/pull/651