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>

beacon-chain/db/kv/BUILD.bazel

@@ -24,6 +24,7 @@ go_library(
         "migration_block_slot_index.go",
         "migration_finalized_parent.go",
         "migration_state_validators.go",
+        "p2p.go",
         "schema.go",
         "state.go",
         "state_summary.go",
@@ -96,6 +97,7 @@ go_test(
         "migration_archived_index_test.go",
         "migration_block_slot_index_test.go",
         "migration_state_validators_test.go",
+        "p2p_test.go",
         "state_summary_test.go",
         "state_test.go",
         "utils_test.go",