Allow customize keep_archive for Uploader and Builder class in config.py

[newgene]

both BaseSourceUploader and DataBuilder have a keep_archive property (default is 10) for the number of old data builds to keep:

biothings.api/biothings/hub/dataload/uploader.py

Line 105 in 621887f

keep_archive = 10 # number of archived collection to keep. Oldest get dropped first.

biothings.api/biothings/hub/databuild/builder.py

Line 59 in 8e0416b

keep_archive = 10 # number of archived collection to keep. Oldest get dropped first.

We should expose them as two separate config parameters for easier customization. Default of 10 is still fine.