relay: refactor relay_initial_join

From now on during initial join memtx engine prepares vclock, raft and
limbo states, it also sends them during memtx_engine_join.

It's done in order to simplify the code of initial join, as in the
consequent commit checkpoint initial join will be introduced and we want
relay code to handle it the same as read-view join without confusing
conditions.

Needed for tarantool/tarantool-ee#741

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 72cc2b3e)