main_phase_timer should be stopped along with SD

[kheaactua]

If main_phase_timer isn't stopped, it might prevent routingmanagerd from quitting gracefully. Thread will be blocked on join().

This is a followup on #559. The issue is a sporative (through rare) SIGTERM on exit from routingmanagerd.

The key signature of the crash appears to be:

#34 vsomeip_v3::routing_manager_impl::expire_subscriptions (this=0x1962493058, _force=false) at implementation/routing/src/routing_manager_impl.cpp:3307 #35 0x0000004ec2948f2c in vsomeip_v3::sd::service_discovery_impl::expire_subscriptions (_error=..., this=0x19624985c8) at implementation/service_discovery/src/service_discovery_impl.cpp:2828

That PR inverted the ownership structure in SD (better explained by @vvcarvalho here.) This change however calls a stop on main_phase_timer during shutdown.