code: def push(id, body)
        before = @wallets.acq(id) { |w| w.exists? ? w.digest : '' }
        after = OpenSSL::Digest::SHA256.new(body).hexdigest
        if before == after
          @log.debug("Duplicate of #{id} ignored #{Size.new(body.length)}")
          return []
        end
        @log.debug("New content for #{id} arrived #{Size.new(body.length)}")
        @entrance.push(id, body)
      end
docs: Returns a list of modifed wallets (as Zold::Id)