Do not wait for slot initialization during exec node start

[gritukan]

Slot is an exec node entity that is used for a single job execution. For example, the number of slots on the exec node limits the maximum number of concurrent jobs on it.

Slot manager (https://github.com/ytsaurus/ytsaurus/blob/main/yt/yt/server/node/exec_node/slot_manager.cpp) is an entity responsible for slot management: it initializes slots when exec node starts and tracks the free slots when jobs start and finish.

When node starts, slot manager initializes all the slots. In CRI job environment in creates slot_count pods which may be slot, since typical slot_count is 4*cpu_count. During the initialization, node reports to scheduler that there are 0 free slots, so no jobs are executed.

This ticket proposes to make initialization one-by-one and async. Slot manager should start slots one-by-one and when a particular slot is ready it becomes accounted in the slot_count reported by scheduler, so it starts to run jobs on the exec node.

https://github.com/ytsaurus/ytsaurus/blob/main/yt/yt/server/node/exec_node/slot_manager.cpp#L247-L250 -- this is the long call that should be split into the number of smaller calls. I propose to split it into Init and InitSlot methods with first initializing job environment without slots and the second one initializes particular slot.