COMMIT_PENDING should be returned if a writer has written data, but
this data may ultimately disappear. Contains a list of strings, format
unspecified, that agents can use to find out, eventually, if their data
has committed. String corresponds to the nth chunk passed to add().
At present, the format is ,
where sinkfilename is the name of a sinkfile, without directory but with
.done suffix, and offset is the last byte of the associated chunk.