Cleanups

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests

Description

This MR contains a number of clean-ups:

• fix missing checks in tests
• fix missing include directories in build rules
• silence deprecation warnings for deprecated fields in Protobuf declarations
• replace obsolete gTest constructs
• fix wrong forward struct declaration
• fix GrpcRetrierFactory not using a parameter passed to it
• fix missing check for protobuf::json::MessageToString result
• fix ignoring errors returned from glibc functions

Validation

Should build and pass tests.