[services] Tunnelbroker - Switching to use composite key in a messageItem entity

Summary:
To avoid possible message UUID collision and speedup batch messages deletion we should switch to using the composite key for the tunnelbroker-messages
table which will consist of messagesID as a Sort Key and toDeviceID as a Partition key.

This diff is a part of the stack and introduces changes to MessageItem database entity.

Linear task: ENG-1362

Test Plan:
We can use the current MessageItem related tests located at services/tunnelbroker/test/DatabaseManagerTest.cpp.
When running yarn test-tunnelbroker-service-in-sandbox test should not fail including MessageItem related tests:

• PutAndFoundMessageItemsStaticDataIsSame
• PutAndFoundMessageItemsGeneratedDataIsSame
• PutAndFoundByReceiverMessageItemsDataIsSame

Reviewers: karol, tomek

Reviewed By: karol, tomek

Subscribers: ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4475