Implement L2 transactions downloader

[torztomasz]

Resolves L2B-1449

[linear]

L2B-1449 Write Live Transaction Downloader

Write an agent that will use LiveTransactionClient from L2B-1442 to constantly check for incoming live transactions or their alternatives. Transactions are indexed by internal ApexId so always ask for subsequent transactions.

Important - locking the table

There will be two processes writing to l2 transactions table: feeder gateway downloader that does it as a part of SyncService and live transaction downloader which is implemented as part of this story. To make sure they don't update the tabe at the same time use a lock:

• once data is downloaded and you're ready to perform database operations, start a new transaction(!) issue: LOCK <l2_transactions_table_name> IN ROW EXCLUSIVE MODE;
• this will keep the table readable but not writable
• do all operations as part of that transaction(!!!!)
• if a second process tries to do the same, it will wait on that LOCK command until the running transaction finishes.

Acceptance Criteria

• The downloader uses a separate async process (not connected with sync done by *SyncService)
• Use locking in both, see the "Important" not above
• When adding live transaction, and transaction with this id already exists in the table, skip it (do nothing)
• Update process that downloads transactions from feeder gateway so that it removes all (live) transactions with IDs that are going to be added by this feeder update.
• Database locking needs to be tested.