first working tl2-only combinator

[hrissan]

Motivation:

1. tl2 functions cannot be in TLO
2. we want to remove TLO and use PHP serialization for all functions

Client mode:
We should not attempt calling C++ serializers, otherwise everything works the same.

Server mode:
we want first to call factory provided to f$rpc_server_fetch_request2, if factory returns null, fallback to f$rpc_server_fetch_request.

We need factory for 2 reasons:

1. because we cannot save_pos/restore_pos from PHP code
2. we need to call internal functions for setting RPC context CurrentTlQuery::get().set_current_tl_function and CurrentRpcServerQuery::get().save