[3pt] add RAFT prefix for all requests related to 'raft'.

[TarantoolBot]

Product: Tarantool
Since: 2.10.0-beta1
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/internals/box_protocol/
SME: @ EvgenyMekhanik

Details

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.
Requested by @EvgenyMekhanik in tarantool/tarantool@6c2eb11.

Probably related to #2310