[fast-reboot] save fast-reboot state into the db

Fixes sonic-net/sonic-buildimage#4006

1. save fast-reboot state into the db(aligned with sonic-net/sonic-buildimage#3741)
2. `sonic-buildimage/files/scripts/syncd.sh` could detect the FAST reboot type by the saved fast-reboot state, and request syncd to fast shutdown.

Author: dawnbeauty

scripts/fast-reboot

@@ -96,6 +96,8 @@ function clear_fast_boot()
 {
     debug "${REBOOT_TYPE} failure ($?) cleanup ..."
 
+    /usr/bin/redis-cli -n 6 del "FAST_REBOOT|system" > /dev/null || /bin/true
+
     /sbin/kexec -u || /bin/true
 
     teardown_control_plane_assistant
@@ -307,6 +309,7 @@ BOOT_TYPE_ARG="cold"
 case "$REBOOT_TYPE" in
     "fast-reboot")
         BOOT_TYPE_ARG=$REBOOT_TYPE
+        /usr/bin/redis-cli -n 6 set "FAST_REBOOT|system" "1" > /dev/null
         trap clear_fast_boot EXIT HUP INT QUIT TERM KILL ABRT ALRM
         ;;
     "warm-reboot")