grpclb: use two-level drop behavior (#3343)

Previously, the round-robin list that the client uses (effective
round-robin list, ERRL) was the received round-robin list (RRRL)
excluding non-READY backends.  Drop and backend entries are in the
same list.

The problem with it is that when not all backends are READY, drop
entries take a larger proportion in ERRL than they do in the RRRL,
resulting a larger drop ratio than intended.

To fix this, we employ a two-list scheme:

- A "drop list" (DL) that is out of the RRRL, with the same size and
  the same number of drop entries.

- A "backend list" (BL) that contains only the backend entries from
  the RRRL, excluding non-READY ones.

For every pick, the client would round-robin on the DL to determine
whether the pick should be dropped.  Only when it's not dropped,
round-robin on the BL to pick the actual backend.

This way, the drop ratio is always equal to the proportion they take
in the RRRL.