[orchagent]:add local_discriminator for BFD session to state_db

[baorliu]

What I did
Add local_discriminator to state DB when bfdorch create_bfd_session.

Why I did it
For upcoming BFD debugging related CLI, user need to use local_discriminator to specify which BFD session they want to debug. current show bfd CLI does not have this information (the local_discriminator is not in state DB).
Also, this is local_discriminator printed into syslog, user may want to correlate this local_discriminator to BFD session.

How I verified it
with modified code, and modified show bfd python script, the local discriminator can be shown in 'show bfd' cmd:
(show "NA" if there is no local_discriminator in state DB.)
modification for "show bfd" script will be committed separately in another PR.

cisco@sonic:/var/tmp$ show bfd sum
Total number of BFD sessions: 1
Peer Addr    Interface    Vrf      State    Type          Local Addr      TX Interval    RX Interval    Multiplier  Multihop    Local Discriminator
-----------  -----------  -------  -------  ------------  ------------  -------------  -------------  ------------  ----------  ---------------------
192.85.2.24  default      default  Down     async_active  192.85.2.29            1000           1000             3  true        NA
cisco@sonic:/var/tmp$ show bfd peer 192.85.2.24
Total number of BFD sessions for peer IP 192.85.2.24: 1
Peer Addr    Interface    Vrf      State    Type          Local Addr      TX Interval    RX Interval    Multiplier  Multihop    Local Discriminator
-----------  -----------  -------  -------  ------------  ------------  -------------  -------------  ------------  ----------  ---------------------
192.85.2.24  default      default  Down     async_active  192.85.2.29            1000           1000             3  true        NA
cisco@sonic:/var/tmp$ docker exec -i swss swssconfig /dev/stdin < bfd.cfg
cisco@sonic:/var/tmp$ show bfd peer 192.85.2.24
Total number of BFD sessions for peer IP 192.85.2.24: 1
Peer Addr    Interface    Vrf      State    Type          Local Addr      TX Interval    RX Interval    Multiplier  Multihop      Local Discriminator
-----------  -----------  -------  -------  ------------  ------------  -------------  -------------  ------------  ----------  ---------------------
192.85.2.24  default      default  Down     async_active  192.85.2.29            1000           1000             3  true                            1
cisco@sonic:/var/tmp$ show bfd sum
Total number of BFD sessions: 1
Peer Addr    Interface    Vrf      State    Type          Local Addr      TX Interval    RX Interval    Multiplier  Multihop      Local Discriminator
-----------  -----------  -------  -------  ------------  ------------  -------------  -------------  ------------  ----------  ---------------------
192.85.2.24  default      default  Down     async_active  192.85.2.29            1000           1000             3  true                            1

Details if related
Example of BFD session entry in state DB with local_discriminator:

"BFD_SESSION_TABLE|default|default|192.85.2.24": {
    "expireat": 1667599670.656455, 
    "ttl": -0.001, 
    "type": "hash", 
    "value": {
      "local_addr": "192.85.2.29", 
      "local_discriminator": "1", 
      "multihop": "true", 
      "multiplier": "3", 
      "rx_interval": "1000", 
      "state": "Down", 
      "tx_interval": "1000", 
      "type": "async_active"
    }
  },

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: baorliu / name: Baorong Liu (97a93f8)

[prsunny]

@baorliu , please sign CLA. Can you also raise the PR to master with a VS test?

[prsunny]

/azp run

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