Remove Buffer.with_capacity() and add Buffer.clear()
Remove Buffer.with_capacity() and add Buffer.clear()

The order of `with_capacity()` matters if used with `with_mem_tracker()`.
It could leak memory if called after `with_mem_tracker()`. This removes
it as the same functionality can be achieved with `reserve()`.

Also add a `clear()` function for `Buffer`.

sunchao/parquet-rs,sunchao/parquet-rs