bug: multiple gitcoin passport downloads at the same time

[lucasmenendez]

Describe the bug
Every gitcoin token downloads its own version of the whole database, including the rest of the stamps.

Current behavior
Every stamp from gitcoin works as an individual token for the scanner. Until the last changes in the scanner, every token shares the same instance of a HolderProvider, which fits perfectly with the gitcoin use case.

The gitcoin holder provider, when inits, start to download the latest scores and updates its internal database to be ready to be query by the scanner. Unfortunately, since the scanner can scan multiples tokens concurrently, the tokens do not share holder providers any more, which produces multiple gitcoin downloads concurrently

Expected behavior
The download of the latest scores must be atomic between tokens of gitcoinspassport type, it can be achieved in multiple layers of the service:

• scanner/providers/manager: Introducing some exceptions for certain providers such as the gitcoinpassport
• scanner/providers/gitcoin: Introducing some logic to manage blocking download between instances.