Epic: Refactor Agent Indexing: better indexing positioning, relayer fast-delivery

[tkporter]

Follows #3281, which is intended to be done ASAP

We should do these next - these are also in chronological order. These are less pressing but after this we'll be in a good indexing situation where we can use alternative modalities and can spin up a relayer that delivers messages without it having indexed all historical data.

Better indexing positioning

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1. Relayer startup is fast even with empty db #3885
   relayer +1
   
2. Refactor relationships between tasks to not rely on reading nonce by nonce from the DB #3269
   agent indexing +2
   
3. Better metrics & logs around indexing health / ETA #3265
   agent indexing +2
   
4. Implement a backward & forward-backward version of a watermarked cursor #3336
   agent indexing +2
   
5. Make block ranges vs sequence ranges less confusing; ensure at compile time that indexer and cursor pairs have consistent range types #3266
   agent indexing +2
   
6. Backward sequence-aware indexing shouldn't ever repeatedly query block range 0..=0 and should handle RPC errors better #3365
   agent bug indexing +3
7. Easily configurable log fetchers #3268
   agent indexing +2
   
8. Write a quick devrel blog post on fast relayer startup without needing historing indexing

related: #2313 & other indexing issues https://github.com/hyperlane-xyz/hyperlane-monorepo/issues?q=is%3Aissue+is%3Aopen+indexing