miner: lower default min miner tip from 1 gwei to 0.001 gwei

[karalabe]

This PR is the result of a long debate as to what the default minimum miner tip expectation of Geth should be. It's important to highlight here that this is just a CLI flag that miners can change, but still people felt strongly that the default should be more reasonable (#29771).

The 1 gwei was originally meant as an opportunity cost for doing more computation on the block + propagating a larger block, which could have someone beat the miner to producing a block. With PoS, there's no opportunity cost wrt someone producing a better block, so it makes sense to reduce the minimum needed price.

Whilst in theory we could also just completely remove it, I feel that miners should still be rewarded something for adding transactions to their blocks, and with 0.001 gwei we're already under the rounding error category. I don't really see a reason to go below this point in the current climate. Maybe in the future if something changes, this can be revisited.

cc @dataalways

[holiman]

LGTM

[dataalways]

lgtm.
If you want to tune the value in the future, you're welcome to consult this query to estimate the effects of different parametrizations.