Allow dustrelayfee to be set to zero

[luke-jr]

I don't see and can't think of any rationale for forbidding this configuration.

[Empact]

GetDustThreshold is defined relative to dustRelayFee, FYI:

    // "Dust" is defined in terms of dustRelayFee,
    // which has units satoshis-per-kilobyte.
    // If you'd pay more in fees than the value of the output
    // to spend something, then we consider it dust.
    // A typical spendable non-segwit txout is 34 bytes big, and will
    // need a CTxIn of at least 148 bytes to spend:
    // so dust is a spendable txout less than
    // 182*dustRelayFee/1000 (in satoshis).
    // 546 satoshis at the default rate of 3000 sat/kB.
    // A typical spendable segwit txout is 31 bytes big, and will
    // need a CTxIn of at least 67 bytes to spend:
    // so dust is a spendable txout less than
    // 98*dustRelayFee/1000 (in satoshis).
    // 294 satoshis at the default rate of 3000 sat/kB.

[morcos]

utACK

[maflcko]

Concept ACK. Might as well fix the braces {}, since this is a tiny code change.

[practicalswift]

utACK 874e818

[laanwj]

I don't see and can't think of any rationale for forbidding this configuration.

Potential rationales, for sanity check:

• Is zero ever used as special value?
• Do we ever divide by this value?

[morcos]

@laanwj I reviewed for those concerns and did not find anything of note. A couple of tests assume there is a dustrelayfee and will break if we change the default to 0, but nothing that needs to be changed for this PR.

[promag]

utACK 874e818.

There is no rationale in #9380 for such condition.