Introduce multidaemon

* Changelog entry
Type: feature
Component:  misc-server

Enables running several server instances (**daemons**) inside a single `ytserver` process (colloquially named **multidaemon**). These deamons will, of course, use a shared pool of singletons but will stay incompletely independent otherwise. Each daemon instance is tagged with a unique **daemon id**. Multiple instances of the same type are possible.

A good usecase for such a feature is local YT or integration testing.

Just start `ytserver-multi --config multi.yson` and enjoy. Here `multi.yson` is something like
```
{
    tcmalloc = { ... };
    tcp_dispatcher = { ... };
    logging = { ... };
    ... (other singleton configs)

    daemons = {
         master_0 = { type = master; config = { .... }; };
         master_1 = { type = master; config = { .... }; };
         master_1 = { type = master; config = { .... }; };
         http_proxy = { type = http_proxy; config = { .... }; };
         scheduler = { type = scheduler; config = { .... }; };
         controller_agent = { type = controller_agent; config = { .... }; };
    };
}
```

The full list of currently supported daemons is as follows:
- `master`
- `node`
- `scheduler`
- `http_proxy`
- `rpc_proxy`
- `cell_balancer`
- `clock`
- `controller_agent`
- `cypress_proxy`
- `discovery_server`
- `kafka_proxy`
- `master_cache`
- `queue_agent`
- `replicated_table_tracker`
- `tablet_balancer`
- `tcp_proxy`
- `timestamp_provider`
commit_hash:640c639a3b56919617b777190b3ffe0142178436

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