iproto: add RAFT prefix for all requests related to 'raft'.

Adding interactive transactions over iproto streamss requires
adding new request types for begin, commit and rollback them.
The type names of these new requests conflict with the existing
names for the 'raft' requests. Adding RAFT prefix for all requests
related to 'raft' resolves this problem.

Part of #5860

@TarantoolBot document
Title: add RAFT prefix for all requests related to 'raft'.
Rename IPROTO_PROMOTE, IPROTO_DEMOTE, IPROTO_CONFIRM and
IPROTO_ROLLBACK to IPROTO_RAFT_PROMOTE, IPROTO_RAFT_DEMOTE,
IPROTO_RAFT_CONFIRM and IPROTO_RAFT_ROLLBACK accordingly.