[services] Tunnelbroker - Fix DeliveryBrokerTest.cpp warnings while building by Clang

Summary:
Fixes build types conversion warnings while building by Clang:

DeliveryBrokerTest.cpp:50:22: non-constant-expression cannot be narrowed from type 'time_t' (aka 'long') to 'uint64_t' (aka 'unsigned long
long') in initializer list [-Wc++11-narrowing]
[build]       .deliveryTag = std::time(0),

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

Reviewers: karol-bisztyga, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3882