All raft functions worked with a global raft object. That would
make impossible to move raft to a separate module, where it could
be properly unit-tested with multiple raft nodes in each test.

The patch adds an explicit raft pointer argument to each raft
function as a first part of moving raft to a separate library.

The global object is renamed to box_raft_global so as to emphasize
this is a global box object, not from the future raft library.

Its access now should go through box_raft() function, which will
get some sanity checks in the next commit.

Part of #5303