vinyl: keep track of range in-memory min lsn
vinyl: keep track of range in-memory min lsn

We need it to check if a range has to be checkpointed. Currently, we
iterate over all attached in-memory indexes to get it. This patch adds
range->min_lsn instead.

rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool