[core] Optimize toArray of all ByteIterators except RandomByteIterator (#1112)

Method nextBuf is a clever hack that outperforms Random.nextBytes
but performs poorly for all other ByteIterator implementations

This commit moves it to RandomByteIterator and adds efficient
toArray implementations for other ByteIterator classes.

Also InputStreamByteIterator.reset method that unconditionally
throws UnsupportedOperationException is fixed