[logging] add post-facto filter

invoking .filter(contexts, name, filter) will apply ‘filter’ to the
object to be logged prior to writing

- context is a dot-separated list of context names (e.g., “ua.http”)

- name is the first parameter to the log call

- filter is either undefined, null, or a function

undefined = delete the filter
null = the null filter (do not log)
function(level, object)