binder: plan not optimal for count(*) with filter

[skyzh]

PhysicalProjection: exprs [InputRef #0]
  PhysicalSimpleAgg: 1 agg calls
    PhysicalTableScan: table #9, columns [0], with_row_handler: true, is_sorted: false, expr: Gt(InputRef #0, Int32(5) (const))

The plan is not optimal. As we already have column 0 scanned, we don't need row handler.

We can refine the binder logic later to handle this case, and get this PR merged at first. Thanks!

Originally posted by @skyzh in #447 (comment)

[skyzh]

Looks better like a task for optimizer... Only optimizer knows how many columns will be read indeed.

[xxchan]

discussions here #503