Issue28 track metadata updates

[ssallam]

Fixes #28

Changes proposed in this PR:

• Add data source for the Metadata contract in all supported networks
• Track/process created/updated events from the Metadata contract
• Link datatoken with all of the corresponding metadata updates

Here is a sample output:

"datatokens": [
      {
        "id": "0x003f89b1b206165029573e5855c5567ce39b5751",
        "metadataUpdateCount": "1",
        "updates": [
          {
            "block": 7443427,
            "timestamp": 1603809123,
            "userAddress": "0x001a08b8b5c4e41fba84f963feee4a6d15528b3e"
          }
        ]
      },
      {
        "id": "0x18366683d873c0e1b5a7f1b9ca8fdbad5ed8177a",
        "metadataUpdateCount": "2",
        "updates": [
          {
            "block": 7496507,
            "timestamp": 1604605855,
            "userAddress": "0xa68f7bae2d4bf3aad6e4f869e2c162ff7bd32281"
          },
          {
            "block": 7494425,
            "timestamp": 1604574622,
            "userAddress": "0xa68f7bae2d4bf3aad6e4f869e2c162ff7bd32281"
          }
        ]
      }
 ]

[mihaisc]

LGTM