text: Title: Profile memory usage
  Body: I have got simple test (with LOOP to reproduce my problem):
  ```
  *** Test Cases ***
  Mem Leak
      Log  Start Sleep ...  console=${True}
      Sleep  3s
      ${l} =  Create List
      FOR  ${i}  IN RANGE  ${30000}
          ${a} =  Set Variable  XXX
      END
      Log  End Test ...  console=${True}
      Sleep  3s
  ```
  I see that robot uses more than 60Mb (between 2 pauses). Log saving i have disabled
  `robot --log None --loglevel TRACE Test.robot`
  
  My initial problem I have a lot of test cases (9k+) and OOM killed my robot process.