vinyl: bump mem version after committing statement

Since commit 1e1c1fdb ("vinyl: make read iterator always return
newest tuple version") vinyl read iterator guarantees that any tuple it
returns is the newest version in the iterator read view. However, if we
don't bump mem version after assigning LSN to a mem statement, a read
iterator using committed_read_view might not see it and return a stale
tuple. Currently, there's no code that relies on this iterator feature,
but we will need it for building new indexes. Without this patch, build
(introduced later in the series) might return inconsistent results.

Needed for #1653