fix: stricter term checks in CaS

Consider the case when a client requests `index: X, term: T`, but
`term(X) != T`. Picodata used to check all entries in the range `[X+1,
last]`, but it didn't check the index `X` itself.

This patch provides this one extra check. Thereby it makes sure that
all entries in the range `[X, last]` belong to the same term `T ==
current_term`.