[services] Tunnelbroker - Add createdAt timestamp to the messageItem

Summary:
While DynamoDB doesn't preserve the insert records order, we can sort the message records by the timestamp of its creation and create
a rule for sorting them inside the DynamoDB to preserve the delivery FIFO order.

Linear task: ENG-1027

Test Plan:
Passed test at D3806.
Run yarn test-tunnelbroker-service all tests are run and succeed.

Reviewers: karol-bisztyga, palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3805