Assorted trivial cleanup of c5c239e26e

Assorted trivial cleanup of c5c239e26e

c5c239e26e made btree vacuum use the read stream API. Though it used
functions declared in read_stream.h, it relied on transitively including
it. Explicitly include that file. Also remove an extraneous newline and
decrease the scope of one of the local variables in btvacuumscan().