Test SPQR Router config reload

[Denchick]

This issue is only about the SPQR Router component.

SPQR Router supports configuration reloading (it reloads config on SIGHUP signal), although there is not enough testing for this feature at the moment.

We need to test (write more feature tests) as many settings as possible in the SPQR router config and document which ones actually support reloading and which are not.

• log_level
• time_quantiles
• maintain_params
• with_jaeger
• pgproto_debug
• log_filename
• availability_zone
• prefer_same_availability_zone
• show_notice_messages
• init_sql
• use_init_sql
• exit_on_init_sql
• default_target_session_attrs
• memqdb_backup_path
• jaeger_url
• frontend_rules
• query_routing
• frontend_tls
• backend_rules
• shards
• schema_cache_backend_rule
• multidb_pool_size
• workload_file
• workload_batch_size
• ignore_cancel
• acceptor_buffer_size
• client_init_max
• use_systemd_notifier
• systemd_notifier_debug

What we defenetely don't want to support:

• daemonize
• host
• router_port
• router_ro_port
• admin_console_port
• grpc_api_port
• world_shard_fallback
• use_coordinator_init
• router_mode
• reuse_port
• with_coordinator