[services] Tunnelbroker - DatabaseManager method to find messages by the receiver

Summary:
We need to fetch undelivered messages from the database by the receiver which is toDeviceID field in our case.
findMessageItemsByReceiever method was implemented for this purpose.

Related linear task: ENG-751

Architecture review for the tunnelbroker.

Test Plan: Unit tests are on D3781

Reviewers: palys-swm, karol-bisztyga

Reviewed By: palys-swm, karol-bisztyga

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

Differential Revision: https://phabricator.ashoat.com/D3239