vinyl: make sure all statements with LSN <= snapshot LSN are dumped

In contrast to the memtx engine, which populates in-memory trees from
Engine::prepare(), in case of Vinyl statements are inserted into
in-memory trees after WAL write, from the Engine::commit() callback.
Therefore, to make sure all statements inserted before snapshot are
dumped, we must initiate checkpoint after WAL rotation. Currently, it is
not true - checkpoint is initiated from Engine::beginCheckpoint(). To
make Vinyl snapshots consistent (not requiring xlog replay), we have to
fix that, so introduce a new callback, Engine::prepareWaitCheckpoint(),
which is called right after WAL rotation, and trigger Vinyl checkpoint
from it.