Use is_write_vectored to optimize the write_vectored implementation for BufWriter

[mzabaluev]

In case when the underlying writer does not have an efficient implementation write_vectored, the present implementation of
write_vectored for BufWriter may still forward vectored writes directly to the writer depending on the total length of the data. This misses the advantage of buffering, as the actually written slice may be small.

Provide an alternative code path for the non-vectored case, where the slices passed to BufWriter are coalesced in the buffer before being flushed to the underlying writer with plain write calls. The buffer is only bypassed if an individual slice's length is at least as large as the buffer.

Remove a FIXME comment referring to #72919 as the issue has been closed with an explanation provided.