CYKLO-1003: log information about created / discarded upload buckets
with DEBUG verbosity.
CYKLO-1003: the form provides upload_bucket_size parameter now which
contains the number of items uploaded asynchronously (items subsequently
removed by the client are subtracted from the count). When invalid
bucket id is presented, but the item count is zero,
attachement_async_upload_failed error will not be reported, as a
workaround.