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
diff --git a/logd/CommandListener.cpp b/logd/CommandListener.cpp
index 031c740..eafa28f 100644
--- a/logd/CommandListener.cpp
+++ b/logd/CommandListener.cpp
@@ -96,8 +96,7 @@
         return 0;
     }
 
-    mBuf.clear((log_id_t) id, uid);
-    cli->sendMsg("success");
+    cli->sendMsg(mBuf.clear((log_id_t) id, uid) ? "busy" : "success");
     return 0;
 }