Fix #3040

- Introduce "notice" log level
- info and notice levels go on stdout, not on stderr
- Label "info:" doesn't get printed on stdout (but does to log file)
Adjust code according to comments on pull request

It remains to substitute the level of some of the messages from `info'
to `notice'.
Add guarantee to format_log_message

Also solve minor compilation errors.
Separate info from notice level messages
Rethink INFO/NOTICE separation based on more comments

Promoted some of the worse situations' messages into WARNING, as per
the discussion on #3040.
Addressing further comments from @timmaxw and @larkost

The agreement should be, as per @timmaxv's latest comment: a message
goes to NOTICE if the answer to the question "Is this message useful
for a user running RethinkDB manually?" is yes.
Print string for console only if not at info level
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next

Conflicts:
	src/clustering/administration/main/serve.cc