This commit introduces `NextKVer` interface which only consists of
a single method for returning KVs from somewhere. The interface is then
used by the `cFetcher`. Currently, there is only one implementation
(`row.KVFetcher`), but soon we'll introduce another one to push the
projections into the KV layer. This required moving
`MVCCDecodingStrategy` type into `storage` package as well.

Note that I chose to not introduce other methods that `cFetcher` is
calling on `row.KVFetcher` since they won't be necessary for the
pushdown work. Also since (at least currently) the `cFetcher` doesn't
use the `spanID` value, we omit it from the interface method signature
(it's only used by the lookup joins which aren't yet vectorized).

Additionally, this commit moves a single comment to the appropriate
place.

Release note: None