raft: check box_raft is inited before usage

Since box_raft is now initialized at runtime and is used from
several subsystems (memtx for snapshots; applier for accepting
rows; box.info for monitoring), it may be easy to screw the
intialization order and accidentally use the not initialized
global raft object.

This patch adds a sanity check ensuring it does not happen. The
raft state is set to 0 at program start. Then any access to the
global raft object firstly checks the state not being 0.

The initialization order will get trickier when raft will stop
using globals from replication and from box, and will be used from
them more extensively.

Part of #5303