[ledd] Use select() with timeout for AppDB notifications (sonic-net#16)

Otherwise ledd ignores signals other than SIGKILL making impossible to
use __del__() destructors in LedClass implementations and delays pmon
docker container shutdown up to 10s.

Signed-off-by: Sergey Popovich <sergey.popovich@ordnance.co>

sonic-ledd/scripts/ledd

@@ -50,6 +50,8 @@ REDIS_HOSTNAME = "localhost"
 REDIS_PORT = 6379
 REDIS_TIMEOUT_USECS = 0
 
+SELECT_TIMEOUT = 1000
+
 #========================== Syslog wrappers ==========================
 
 def log_info(msg):
@@ -204,7 +206,13 @@ def main():
 
     # Listen indefinitely for changes to the PORT table in the Application DB
     while True:
-        (state, c) = sel.select()
+        # Use timeout to prevent ignoring the signals we want to handle
+        # in signal_handler() (e.g. SIGTERM for graceful shutdown)
+        (state, c) = sel.select(SELECT_TIMEOUT)
+
+        if state == swsscommon.Select.TIMEOUT:
+            # Do not flood log when select times out
+            continue
         if state != swsscommon.Select.OBJECT:
             log_warning("sel.select() did not return swsscommon.Select.OBJECT")
             continue