MINOR: log: support extra log origins for '%OG' alias

MINOR: log: support extra log origins for '%OG' alias

Following previous commits, let's improve log_orig_to_str() so that
extra log origins (registered through log_orig_register()) can be
translated to string from origin ID.

For that, it is required to add eb_32 tree node to log_origin struct in
order to enable quick integer lookup during runtime. Slow name lookup
using the list is acceptable for config parsing, but it is not the case
during runtime when log_orig_to_str() is expected to be used. Also, to
prevent duplicated info, get rid of ->id field and use ->tree.key instead