values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    uint8_t n;
    
    BOOST_CHECK(ParseUInt8("255", nullptr));
    BOOST_CHECK(ParseUInt8("0", &n) && n == 0);
    BOOST_CHECK(ParseUInt8("255", &n) && n == 255);
    BOOST_CHECK(ParseUInt8("0255", &n) && n == 255); 
    BOOST_CHECK(ParseUInt8("255", &n) && n == static_cast<uint8_t>(255));
    BOOST_CHECK(ParseUInt8("+255", &n) && n == 255);
    BOOST_CHECK(ParseUInt8("00000000000000000012", &n) && n == 12);
    BOOST_CHECK(ParseUInt8("00000000000000000000", &n) && n == 0);
    
    BOOST_CHECK(!ParseUInt8("-00000000000000000000", &n));
    BOOST_CHECK(!ParseUInt8("", &n));