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

BOOST_CHECK_EQUAL(amount, 110000000LL);
    BOOST_CHECK(ParseFixedPoint("1.10000000000000000", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 110000000LL);
    BOOST_CHECK(ParseFixedPoint("1.1e1", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 1100000000LL);
    BOOST_CHECK(ParseFixedPoint("1.1e-1", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 11000000LL);
    BOOST_CHECK(ParseFixedPoint("1000", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 100000000000LL);
    BOOST_CHECK(ParseFixedPoint("-1000", 8, &amount));
    BOOST_CHECK_EQUAL(amount, -100000000000LL);
    BOOST_CHECK(ParseFixedPoint("0.00000001", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 1LL);
    BOOST_CHECK(ParseFixedPoint("0.0000000100000000", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 1LL);
    BOOST_CHECK(ParseFixedPoint("-0.00000001", 8, &amount));
    BOOST_CHECK_EQUAL(amount, -1LL);
    BOOST_CHECK(ParseFixedPoint("1000000000.00000001", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 100000000000000001LL);
    BOOST_CHECK(ParseFixedPoint("9999999999.99999999", 8, &amount));
    BOOST_CHECK_EQUAL(amount, 999999999999999999LL);
    BOOST_CHECK(ParseFixedPoint("-9999999999.99999999", 8, &amount));
    BOOST_CHECK_EQUAL(amount, -999999999999999999LL);
    BOOST_CHECK(!ParseFixedPoint("", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("a-1000", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-a1000", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-1000a", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("-01000", 8, &amount));
    BOOST_CHECK(!ParseFixedPoint("00.1", 8, &amount));