MINOR: log: introduce extra log profile steps

MINOR: log: introduce extra log profile steps

add a way to register additional log origins using log_origin_register()
that may be used as log profile steps from log profile sections.

For now this does nothing as no extra origins are registered and extra log
origins are not yet considered for runtime logging paths.

When specifying an extra logging step for on <step> under log-profile
section, the logging step is stored within a binary tree for efficient
lookup during runtime. No performance impact should be expected if extra
log origins are not being used, and slight performance impact if extra
log origins are used.

Don't forget to update the documentation when new log origins are added
(both %OG log alias and on <step> log-profile keyword are concerned.