make Fast USDC SeenTxs prunable

[turadg]

What is the Problem Being Solved?

We had planned to use blockTimestamp to prune old transactions, but that's too expensive to get from the OCW. (@jacquesvcritien reports blockNumber comes in the message that they’re watching for while blockTimestamp requires a follow up RPC call.)

As of #11049 we record blockNumber instead, which is counted differently across source chains.

Description of the Design

A way to prune "old" transactions without querying vstorage.

More TBD

Security Considerations

Scaling Considerations

Test Plan

Upgrade Considerations

[dckc]

for reference:

• include blockNumber in SeenTxs records #10992
• make SeenTxs prunable #10993

[turadg]

We're back to getting real blockTimestamp, meaning no new work to satisfy this issue. I'll leave it open until we've actually done pruning.