raft: move box_raft_* to src/box/raft.h and .c

The commit moves raft functions and objects specific for box to
src/box/raft from src/box/box and src/box/raftlib.

The goal is to gradually eliminate all box dependencies from
src/box/raftlib and move it to src/lib/raft.

It makes the compilation work again after the previous commit
broke it.

Part of #5303