[services] Tunnelbroker - Switch to milliseconds at createdAt in messages

Summary:
Switch to milliseconds from unixtime seconds for the createdAt field on Tunnelbroker database message item related to the
ENG-1027 comment.

Using the tools::getCurrentTimestamp() to get the milliseconds since epoch.

Related task: ENG-1045

Test Plan: Run yarn test-tunnelbroker-service to build and run the tests.

Reviewers: karol-bisztyga, palys-swm, varun, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3880