relay: join new replicas off read view

Historically, we join a new replica off the last checkpoint. As a
result, we must always keep the last memtx snapshot and all vinyl data
files corresponding to it. Actually, there's no need to use the last
checkpoint for joining a replica. Instead we can use the current read
view as both memtx and vinyl support it. This should speed up the
process of joining a new replica, because we don't need to replay all
xlogs written after the last checkpoint, only those that are accumulated
while we are relaying the current read view. This should also allow us
to avoid creating a snapshot file on bootstrap, because the only reason
why we need it is allowing joining replicas. Besides, this is a step
towards decoupling the vinyl metadata log from checkpointing in
particular and from xlogs in general.

Closes #1271