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(v5[2].copies, 2);
    BOOST_CHECK_EQUAL(v5[3].copies, 1);
    auto v6 = Cat(std::move(v1), v3);
    BOOST_CHECK_EQUAL(v6.size(), 3U);
    BOOST_CHECK(v6[0].origin == &t1);
    BOOST_CHECK(v6[1].origin == &t1);
    BOOST_CHECK(v6[2].origin == &t2);
    BOOST_CHECK_EQUAL(v6[0].copies, 1);
    BOOST_CHECK_EQUAL(v6[1].copies, 2);
    BOOST_CHECK_EQUAL(v6[2].copies, 1);
    auto v7 = Cat(v2, std::move(v4));
    BOOST_CHECK_EQUAL(v7.size(), 4U);
    BOOST_CHECK(v7[0].origin == &t2);
    BOOST_CHECK(v7[1].origin == &t1);
    BOOST_CHECK(v7[2].origin == &t2);
    BOOST_CHECK(v7[3].origin == &t3);
    BOOST_CHECK_EQUAL(v7[0].copies, 1);
    BOOST_CHECK_EQUAL(v7[1].copies, 1);
    BOOST_CHECK_EQUAL(v7[2].copies, 1);
    BOOST_CHECK_EQUAL(v7[3].copies, 0);
    auto v8 = Cat(std::move(v2), std::move(v3));
    BOOST_CHECK_EQUAL(v8.size(), 3U);
    BOOST_CHECK(v8[0].origin == &t2);
    BOOST_CHECK(v8[1].origin == &t1);
    BOOST_CHECK(v8[2].origin == &t2);
    BOOST_CHECK_EQUAL(v8[0].copies, 0);
    BOOST_CHECK_EQUAL(v8[1].copies, 1);
    BOOST_CHECK_EQUAL(v8[2].copies, 0);
}
BOOST_AUTO_TEST_CASE(message_sign)