Use smaller execution thresholds for exex backfill impl

[mattsse]

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Describe the feature

currently BackfillJobFactory uses:

https://github.com/paradigmxyz/reth/blob/6d57b9eaac0c093feb453f8f4d2bef90e28e8993/crates/exex/exex/src/backfill/factory.rs#L28-L28

the default execution stage parameters, but we don't disable mdbx timeouts, so this could exceed the allowed timeout.

TODO

• use more appropriate execution threshold parameters

an alternative would be always using single block execution for this

Additional context

No response