Fix file contract element bug

[chris124567]

Account for multiple revisions of the same contract in the same block. If we only look at the data from ForEachFileContractElement we will only get the latest revision which will result in missing data in fcDBIDs (the map that connects file contract IDs and their revision numbers to the file_contract_elements id in the database), which will cause addFileContracts or addFileContractRevisions to fail with "missing dbID". For example in block 22051: https://siascan.com/tx/293ba9948c6bf4ac4b639a6326421be234b73279934a9a0a4feda7012ca80722 and https://siascan.com/tx/60d7b56390e373c67066cb440f1821f46b2dd245e134556eaf4fac472a58d6e9

[ChrisSchinnerl]

Since this PR specifically targets a bug to fix I'd include a regression test to both verify the issue existed and to make sure it is fixed now.

[chris124567]

Since this PR specifically targets a bug to fix I'd include a regression test to both verify the issue existed and to make sure it is fixed now.

When I comment out either or both of the following loops in addFileContractElements which were introduced in this PR the newly added test fails with the same error mentioned in the PR description.

        for j, fc := range txn.FileContracts {
            fcID := txn.FileContractID(j)
            dbFC := explorer.DBFileContract{ID: txn.FileContractID(j), RevisionNumber: fc.RevisionNumber}
            if _, exists := fcDBIds[dbFC]; exists {
                continue
            }

            if err := addFC(fcID, 0, fc, false, true, false); err != nil {
                return nil, fmt.Errorf("addFileContractElements: %w", err)
            }
        }
        for _, fcr := range txn.FileContractRevisions {
            fc := fcr.FileContract
            dbFC := explorer.DBFileContract{ID: fcr.ParentID, RevisionNumber: fc.RevisionNumber}
            if _, exists := fcDBIds[dbFC]; exists {
                continue
            }

            if err := addFC(fcr.ParentID, 0, fc, false, true, false); err != nil {
                return nil, fmt.Errorf("addFileContractElements: %w", err)
            }
        }

And the test passes so I think this is good. I had to debug this odd issue with the Payout field in revisions which took longer than it should have 😄