Changing timestamp type to int64

[staheri14]

Following the discussion in vacp2p/rfc#448, the decision has been made to use int64 type to represent timestamp values across wakuv2 protocols where the time values going to indicate the Unix epoch time in milliseconds. This includes

• timestamp field of WakuMessage.
• senderTime and receiverTime fields of Index object used for pgaination in the store protocol.

This change will touch the following parts of the codebase (non-exclusive):

• Waku store protocol
• WakuMessage
• Message store interface: https://github.com/status-im/nim-waku/blob/master/waku/v2/node/storage/message/message_store.nim
• Waku message store and the Message table schema: https://github.com/status-im/nim-waku/blob/master/waku/v2/node/storage/message/waku_message_store.nim
• A subset of wakunode2 JSON RPC APIs: https://github.com/status-im/nim-waku/tree/master/waku/v2/node/jsonrpc

Acceptance Criteria

• The codebase should be refactored to accommodate this change (making sure all the tests pass successfully)
• The corresponding specs i.e., Waku2-Store and Waku2-Message should reflect this change
• Changelog should have an entry briefly explaning this update
• A migration script should be added to migration scripts directory to handle the schema update in the Message table