engine: introduce stubs for checkpoint FETCH_SNAPSHOT

This commit introduces engine stubs that enable a new method
of fetching snapshots for anonymous replicas. Instead of using
the traditional read-view join approach, this update allows
file snapshot fetching. Note that file snapshot fetching
is only available in Tarantool EE.

Checkpoint fetching is done via IPROTO_IS_CHECKPOINT_JOIN,
IPROTO_CHECKPOINT_VCLOCK and IPROTO_CHECKPOINT_LSN fields.

If IPROTO_CHECKPOINT_JOIN is set to true, join will be done from
files: .snap for memtx, .run for vinyl, if false - from read view.

Checkpoint join allows to continue from the place, where client
stopped in case of snapshot fetching error. This allows to avoid
rebootstrap of an anonymous client. This can be done by specifying
CHECKPOINT_VCLOCK, which says from which file server should continue
join, client gets vclock at the beginning of the join. Specifying
CHECKPOINT_LSN allows to continue from some position in checkpoint.
Server sends all data >= CHECKPOINT_LSN.

If CHECKPOINT_VCLOCK is not specified, fetching is done from the latest
available checkpoint. If CHECKPOINT_LSN is not specified - start from
the beginning of the snap. So, specifying only IS_CHECKPOINT_JOIN
triggers fetching the latest checkpoint from files.

Needed for tarantool/tarantool-ee#741

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NO_TEST=ee
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