Use an MTU-sized buffer for socket access.

Previously we attempted to avoid buffers in some situations
and create aggressive buffers in other situations. This was
a bad policy, and meant we had some subtle performance bugs.
The one that prompted this is that chunked uploads make
separate network writes for the chunk size, chunk, and newline
separators.

This avoids that problem and the corresponding complexity.
Unfortunately getting the MTU isn't a standard API until
Java 6 / Gingerbread. I tested my own networks and saw 1500
in use (for 3G and WiFi) and 1400 (for VPN over WiFi).
Merge pull request #152 from square/jwilson/mtu

Use an MTU-sized buffer for socket access.