logd: clear return and deal with busy if readers locked

- Propagate to caller the clearing errors, busy blocked by reader.
- For clear, perform retries within logd with a one second lul each,
  telling readers to skip, but on final retry to kill all readers if
  problem still persists due to block reader (or high volume logspammer).

Bug: 23711431
Change-Id: Ie4c46bc9480a7f49b96a81fae25a95c603270c33