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(LockDirectory(dirname, lockname), true);
    
    bool threadresult;
    std::thread thr(TestOtherThread, dirname, lockname, &threadresult);
    thr.join();
    BOOST_CHECK_EQUAL(threadresult, true);
#ifndef WIN32
    
    char ch;
    BOOST_CHECK_EQUAL(write(fd[1], &LockCommand, 1), 1);
    BOOST_CHECK_EQUAL(read(fd[1], &ch, 1), 1);
    BOOST_CHECK_EQUAL((bool)ch, false);
    
    ReleaseDirectoryLocks();
    
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname, true), true);
    
    BOOST_CHECK_EQUAL(write(fd[1], &LockCommand, 1), 1);
    BOOST_CHECK_EQUAL(read(fd[1], &ch, 1), 1);
    BOOST_CHECK_EQUAL((bool)ch, true);
    
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname, true), false);
    
    BOOST_CHECK_EQUAL(write(fd[1], &UnlockCommand, 1), 1);
    BOOST_CHECK_EQUAL(read(fd[1], &ch, 1), 1);
    BOOST_CHECK_EQUAL((bool)ch, true);
    
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname, true), true);