Filterx: add drop/done

[OverOrion]

Example config

log {

  source { example-msg-generator(num(1) template("bar")); };
  source { example-msg-generator(num(1) template("foo")); };
  filterx {
    if ($MSG =~ '.*foo.*' ){
      drop;
    }
    else {
      done;
      declare var_should_not_exist = json(); # This will be skipped
    };

  };
  filterx {
    vars();
  };
  destination { file("/dev/stdout"); };
};

Result

Setting value; name='MESSAGE', value='-- Generated message. --', type='string', msg='0x6500309ffa40', rcptid='28'
Setting value; name='MESSAGE', value='bar', type='string', msg='0x6500309ffa40', rcptid='28'
Incoming generated message; msg='bar'
>>>>>> Source side message processing begin; location='./etc/drop_msg.conf:5:12', msg='0x6500309ffa40', rcptid='28'
Setting value; name='HOST_FROM', value='orion-T14G3', type='string', msg='0x6500309ffa40', rcptid='28'
Setting value; name='HOST', value='orion-T14G3', type='string', msg='0x6500309ffa40', rcptid='28'
Setting tag; name='.source.#anon-source0', value='1', msg='0x6500309ffa40'
Setting value; name='SOURCE', value='#anon-source0', type='string', msg='0x6500309ffa40', rcptid='28'
>>>>>> filterx rule evaluation begin; rule='#anon-filter0', location='./etc/drop_msg.conf:7:10', msg='0x6500309ffa40', rcptid='28'
FILTERX CONDF; expr='./etc/drop_msg.conf:8:9|      $MSG =~ \'.*foo.*\'', value='false', truthy='0', type='boolean'
FILTERX ESTEP; expr='./etc/drop_msg.conf:12:7|     done', value='true', truthy='1', type='boolean'
<<<<<< filterx rule evaluation result; result='matched', rule='#anon-filter0', location='./etc/drop_msg.conf:7:10', dirty='1', msg='0x6500309ffa40', rcptid='28'
>>>>>> filterx rule evaluation begin; rule='#anon-filter1', location='./etc/drop_msg.conf:17:10', msg='0x6500309ffa40', rcptid='28'
FILTERX ESTEP; expr='./etc/drop_msg.conf:18:5|     vars()', value='{"MESSAGE":"bar"}', truthy='1', type='json_object'
<<<<<< filterx rule evaluation result; result='matched', rule='#anon-filter1', location='./etc/drop_msg.conf:17:10', dirty='1', msg='0x6500309ffa40', rcptid='28'
Filterx sync: variable in scope and message in sync, not doing anything; variable='MESSAGE'
Initializing destination file writer; template='/dev/stdout', filename='/dev/stdout', symlink_as='(null)'
affile_open_file; path='/dev/stdout', fd='13'
<<<<<< Source side message processing finish; location='./etc/drop_msg.conf:5:12', msg='0x6500309ffa40', rcptid='28'
Setting value; name='MESSAGE', value='-- Generated message. --', type='string', msg='0x650030a02fd0', rcptid='29'
Setting value; name='MESSAGE', value='foo', type='string', msg='0x650030a02fd0', rcptid='29'
Incoming generated message; msg='foo'
>>>>>> Source side message processing begin; location='./etc/drop_msg.conf:6:12', msg='0x650030a02fd0', rcptid='29'
Setting value; name='HOST_FROM', value='orion-T14G3', type='string', msg='0x650030a02fd0', rcptid='29'
Setting value; name='HOST', value='orion-T14G3', type='string', msg='0x650030a02fd0', rcptid='29'
Setting tag; name='.source.#anon-source1', value='1', msg='0x650030a02fd0'
Setting value; name='SOURCE', value='#anon-source1', type='string', msg='0x650030a02fd0', rcptid='29'
>>>>>> filterx rule evaluation begin; rule='#anon-filter0', location='./etc/drop_msg.conf:7:10', msg='0x650030a02fd0', rcptid='29'
FILTERX CONDT; expr='./etc/drop_msg.conf:8:9|      $MSG =~ \'.*foo.*\'', value='true', truthy='1', type='boolean'
FILTERX ESTEP; expr='./etc/drop_msg.conf:9:7|      drop', value='true', truthy='1', type='boolean'
<<<<<< filterx rule evaluation result; result='explicitly dropped', rule='#anon-filter0', location='./etc/drop_msg.conf:7:10', dirty='1', msg='0x650030a02fd0', rcptid='29'
Window size adjustment; old_window_size='99', window_size_increment='1', suspended_before_increment='FALSE', last_ack_type_is_suspended='FALSE'
<<<<<< Source side message processing finish; location='./etc/drop_msg.conf:6:12', msg='0x650030a02fd0', rcptid='29'
Outgoing message; message='Sep  2 18:40:03 orion-T14G3 bar\x0a'
Sep  2 18:40:03 orion-T14G3 bar

``

[alltilla]

Also, please add some light tests. Thanks!