Since WriterFrontend objects are looked up internally by writer type and
path, and they also expect to write consistent field arguments, it could
be the case that more than one filter of a given stream attempts to
write to the same path (derived either from $path or $path_func fields
of the filter) with the same writer type. This won't work, so now
WriterFrontend objects are bound to the filter that instantiated them so
that we can warn about other filters attempting to write to the
conflicting writer/path and the write can be skipped. Remote logs don't
appear to suffer the same issue due to pre-filtering.
Addresses #842.
