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

{
    fs::path dirname = m_args.GetDataDirPath() / "lock_dir";
    const std::string lockname = ".lock";
#ifndef WIN32
    
    
    
    void (*old_handler)(int) = signal(SIGCHLD, SIG_DFL);
    
    
    
    int fd[2];
    BOOST_CHECK_EQUAL(socketpair(AF_UNIX, SOCK_STREAM, 0, fd), 0);
    pid_t pid = fork();
    if (!pid) {
        BOOST_CHECK_EQUAL(close(fd[1]), 0); 
        TestOtherProcess(dirname, lockname, fd[0]);
    }
    BOOST_CHECK_EQUAL(close(fd[0]), 0); 
#endif
    
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname), false);
    fs::create_directories(dirname);
    
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname, true), true);
    
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname), true);