stats: added stats reporting to the upload helper

stats: added stats reporting to the upload helper

adds a stats_producer for the upload helper, which provides a series of counters
to the stats gatherer, under the name 'chk_upload_helper'.

it examines both the 'incoming' directory, and the 'encoding' dir, providing
inc_count inc_size inc_size_old enc_count enc_size enc_size_old, respectively
the number of files in each dir, the total size thereof, and the aggregate
size of all files older than 48hrs