Use usize to represent Limit::skip

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

pub struct Limit {
    /// Number of rows to skip before fetch
    pub skip: Option<usize>,
    /// Maximum number of rows to fetch
    pub fetch: Option<usize>,
    /// The logical plan
    pub input: Arc<LogicalPlan>,
}

Since skip = 0 and skip = None have same meaning, we can use pure usize to represent skip to reduce complexity.

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Additional context
Find this will work on #3355

[HaoYang670]

cc @alamb @andygrove @Dandandan .
Need your reviews, because this will cause some public API changes.

The reasons I want to do this are:

1. skip = None and skip = Some(0) are equivalent (forall plan, replacing skip = None with skip = 0 doesn't affect the result), so we don't need both of them.
2. This can avoid bugs in corner cases. While working on limit_push_down, I find there are lots of 'map_or, unwrap_or, ' and pattern matching with return, which is difficult to read and easy to have bug
3. We don't need to test skip=None, just to test skip=0 is enough.
4. Actually, I was a little confused about the meaning of skip = None, and fetch = None. And I misunderstood that skip = None meant skipping all rows (because fetch = None means fetching all rows). I am not sure if other users/developers have ever had this confusion.

[alamb]

I think this proposal makes sense to me as well. I can't think of any issues with it