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>

config/features/deprecated_flags.go

@@ -108,6 +108,11 @@ var (
 		Usage:  deprecatedUsage,
 		Hidden: true,
 	}
+	deprecatedP2PMetadata = &cli.StringFlag{
+		Name:   "p2p-metadata",
+		Usage:  deprecatedUsage,
+		Hidden: true,
+	}
 )
 
 // Deprecated flags for both the beacon node and validator client.
@@ -130,6 +135,7 @@ var deprecatedFlags = []cli.Flag{
 	deprecatedEnableQuic,
 	deprecatedAttestTimely,
 	deprecatedDisableExperimentalState,
+	deprecatedP2PMetadata,
 }
 
 var upcomingDeprecation = []cli.Flag{