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:

https://github.com/biothings/biothings.api/blob/621887f04aae13c3a775aea9aa7daacb92ae7ef0/biothings/hub/dataload/uploader.py#L105

https://github.com/biothings/biothings.api/blob/8e0416b95661b8539d66d4978e859aef005b7ae5/biothings/hub/databuild/builder.py#L59

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