server: improve ListPath's memory consumption with batched send

With a lot of paths (hundreds of thousands) gobgp may oom or
stuck in swapping.
Allow to specify max batch size via grpc and keep unlimited batch
size by default since 21093fb
without preallocation on the first run, so it still should not
affect perfomance/allocations with small ammount of paths.

Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>