Buffer overflow output stream

Buffer overflow output stream

Most callers/users of TemporaryBuffer are sizing the in-memory
portion large enough that most outputs fit into RAM. With this
assumption they don't pay close attention to the size of IOs
being written, as it "should" just be a copy from one byte array
to another.

Overflow sets up a local file handle, which is costly to write to
for small IO units. Wrap the local file in a BufferedOutputStream
to combine small writes together. Larger writes can still bypass the
buffer as BOS automatically avoids copying for larger writes.

Change-Id: I09f4136dd65c48830cfda86d9101bc647581018a