Handle logd reconnect.

When statsd reconnects to logd, statsd will read all logs from buffer again. To prevent us from
reprocessing old events, we do the following:

1. At any given moment, record the largest timestamp(T_max) and last timestamp (check point) that
   we've seen before.
2. When reconnection happens, we look for the check point until we see a new log with a timestamp
   larger than T_max.
   -> If we found the CP, resume after the CP. Success
   -> If we can't find CP, there is definitely log loss. We reset all configs.

Note:
1. Logd has an API to read logs after a certain timestamp. But this api is vulnerable to
time changes from Settings. So we cannot rely on it.

2. If logd inserts a new log (with older timestamp) before CP, we cannot detect it. It's not
   possible to detect it without record all timestamps we have seen.

Test: statsd_test
Bug: 77813113

Change-Id: Ic3fdb47230807606ab11dc994cb162194adb8448