raft: move sources to raftlib.h/.c

The commit renames raft.h and raft.c to raftlib.h and raftlib.c.
This is done to prepare to raft split into src/box/ and
src/lib/raft.

The commit is not atomic, the build won't work here. Because if
raft is renamed to raftlib, and in the same commit new raft.c and
raft.h are added, git thinks the original file was changed, and
ruins all the git history.

By splitting move of raft to raftlib and introduction of box/raft
into 2 commits the git history is saved.

Part of #5303

src/box/CMakeLists.txt

@@ -169,6 +169,7 @@ add_library(box STATIC
     port.c
     txn.c
     txn_limbo.c
+    raftlib.c
     raft.c
     box.cc
     gc.c