fix(command_manager): avoid using std::remove to erase elements from std::vector

[zhangyifan27]

On account of std::remove could not actually erase elements from containers,

The removal is done by replacing the elements that compare equal to val by the next element that does not, ... [1]

we may delete command_instance * repeatly when destroy command_manager.

void command_manager::deregister_command(dsn_handle_t handle)
{
    auto c = reinterpret_cast<command_instance *>(handle);
    dassert(c != nullptr, "cannot deregister a null handle");
    utils::auto_write_lock l(_lock);
    for (const std::string &cmd : c->commands) {
        ddebug("unregister command: %s", cmd.c_str());
        _handlers.erase(cmd);
    }
    std::remove(_commands.begin(), _commands.end(), c);	
    delete c;	
}

command_manager::~command_manager()
{	
    for (command_instance *c : _commands) {	
        delete c;	
    }	
}

The correct way to remove command_instance from the vector should be:

    auto iter = std::remove(_commands.begin(), _commands.end(), c);
    _comands.erase(iter, _commands.end());
    delete c;	

This patch remove std::vector<command_instance *> _commands from class command_manager because we already have std::map<std::string, command_instance *> _handlers, and also improve the use of register_command and deregister_command.

[1] https://www.cplusplus.com/reference/algorithm/remove/

[neverchanje]

I would recommend wrapping command_instance* into a RAII class, so that we do not need to UNREGISTER_VALID_HANDLER manually. If we don't refactor in this PR, I will take this later.