Add timestamps to TransactionServer

[ddimaria]

In order to be able to determine the age of a file in pubsub, we need to have timestamps added to messages in the queue. Currently, TransactionServer is:

pub struct TransactionServer {
    pub id: Uuid,
    pub file_id: Uuid,
    pub operations: Vec<u8>,
    pub sequence_num: u64,
}

The proposed change:

pub struct TransactionServer {
    pub id: Uuid,
    pub file_id: Uuid,
    pub operations: Vec<u8>,
    pub sequence_num: u64,
    pub timestamp: DateTime<Utc>,
}