### Summary
Tracking issue: https://github.com/wormhole-foundation/wormhole-explorer/issues/415
When the `tx-tracker` service processes a VAA, sometimes the transaction is not finalized (e.g.:for chains with slow finality, VAAs with low consistency level, out-of-sync nodes). This problem was observed on chains like Solana, Ethereum and Polygon, but could have happened on other chains.
This pull request adds the logic needed to wait and retry processing the VAA, hoping that the originating transaction reaches finality.
Also, the `tx-tracker` service now uses a worker pool to process transactions, which will lead to higher throughput. This will benefit the WormholeScan UI, specially in the case when a large number of VAAs are emitted in a short period of time.
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