Fix too many log lines produced by streaming replication check.

Fix too many log lines produced by streaming replication check.

The process started to call
get_pg_backend_status_from_leader_wd_node() which unconditionally emits
log message:

LOG:  received the get data request from local pgpool-II on IPC interface

LOG:  get data request from local pgpool-II node received on IPC interface is forwarded to leader watchdog node

every sr_check_period seconds, which is annoying. To fix this, an elog
line in process_IPC_data_request_from_leader() is downgraded from LOG
to DEBUG1.

Reported-by: Bo Peng.