engine: let other engines reuse memtx checkpoint and join

The implementation of the checkpoint and join memtx engine methods
is actually generic: all they need is a read view. Let's allow
other engines that support creation of a consistent read view to be
checkpointed and joined by the memtx engine by setting the corresponding
engine flags.

Currently, there's no such engines, i.e. the only engine setting the new
flags is memtx itself. We will use this feature for new engines.

Needed for tarantool/tarantool-ee#617

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