[PATCH v2 0/7] Join replicas off the current read view

[Vladimir Davydov]

Pushed the following patches approved by Kostja to master and
rebased the branch:

>   vinyl: don't pin index for iterator lifetime
>   vinyl: get rid of vy_env::join_lsn
>   memtx: use ref counting to pin indexes for snapshot
>   memtx: enter small delayed free mode from snapshot iterator
>   space: get rid of apply_initial_join_row method

The following patches are still pending review/discussion.

>   vinyl: don't exempt dropped indexes from dump and compaction
>   relay: join new replicas off read view