Replace AvaticaUtils.readFully(InputStream)'s use of ByteArrayOutputStream

I remember reading a ticket a long time ago (ACCUMULO-1000) about how use of ByteArrayOutputStream can be sub-optimal for low-latency calls due to the synchronization around the write method.

While ByteArrayOutputStream gets the job done, an unsynchronized buffer is sufficient for reading the bytes of the HTTP body off of the wire and into a byte array. We don't need the synchronization guarantees that come with BAOS.