Persist metadata sequence number using Beacon DB (#15554)

* Add entry for sequence number in chain-metadata bucket & Basic getter/setter

* Mark p2p-metadata flag as deprecated

* Fix metaDataFromConfig: use DB instead to get seqnum

* Save sequence number after updating the metadata

* Fix beacon-chain/p2p unit tests: add DB in config

* Add changelog

* Add ReadOnlyDatabaseWithSeqNum

* Code suggestion from Manu

* Remove seqnum getter at interface

---------

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

cmd/flags.go

@@ -158,12 +158,6 @@ var (
 		Usage: "Enables the peer id of the node to be fixed by saving the generated network key to the default key path.",
 		Value: false,
 	}
-	// P2PMetadata defines a flag to specify the location of the peer metadata file.
-	P2PMetadata = &cli.StringFlag{
-		Name:  "p2p-metadata",
-		Usage: "The file containing the metadata to communicate with other peers.",
-		Value: "",
-	}
 	// P2PMaxPeers defines a flag to specify the max number of peers in libp2p.
 	P2PMaxPeers = &cli.IntFlag{
 		Name:  "p2p-max-peers",