raft: stop using replicaset.vclock

Raft is being moved to a separate library in src/lib. It means,
it can't depend on anything from box/.

The patch makes raft stop using replicaset.vclock.

Instead, it has a new option 'vclock'. It is stored inside struct
raft by pointer and should be configured using raft_cfg_vclock().

Box configures it to point at replicaset.vclock like before. But
now raftlib code does not depend on it explicitly.

Vclock is stored in Raft by pointer instead of by value so as not
to update it for each transaction. It would be too high price to
pay for raft independence from box.

Part of #5303