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

{
    char ch;
    while (true) {
        int rv = read(fd, &ch, 1); 
        assert(rv == 1);
        switch(ch) {
        case LockCommand:
            ch = LockDirectory(dirname, lockname);
            rv = write(fd, &ch, 1);
            assert(rv == 1);
            break;
        case UnlockCommand:
            ReleaseDirectoryLocks();
            ch = true; 
            rv = write(fd, &ch, 1);
            assert(rv == 1);
            break;
        case ExitCommand:
            close(fd);
            exit(0);
        default:
            assert(0);
        }
    }
}
#endif
BOOST_AUTO_TEST_CASE(test_LockDirectory)