taskd: several important bug fixes

bug fix on some not so easy to reproduce bugs, mostly when running several
tasks in a row and those tasks finish very fast. Fixed:

* bug when multiple msgs were in the buffer waiting to be handled, on
  function read_freezed_msg()

* now it doesn't try to request new msgs when executing the maximum number
  of exec children

* bug on http_child not receiving the correct number of exec children from
  master after SIGCHLD