Incorporated comments from the second iteration

1) Changed package download stats packet to be a key-value pair instead of
just a flat list of values
2) Also, added columns names to the select query against DB
3) Added a try-catch on the CommitCatalog to catch all exceptions and
always open the CatalogWriterGate
Implemented catalog generation for metrics service with multiple instances

1) Each instance of the metrics service website creates a catalog with a
fixed prefix of website_instance_id
2) Also, creates a file called indexes/<website_instance_id>.txt for the
catalog collector to use

P.S: Removed blob directory function and split that into 2