Do not kill connections on recv buffer overflow

Instead of killing a connection when the receive buffer overflows,
just temporarily halt receiving before that happens. Also, no
matter what, always allow at least one full message in the receive
buffer (otherwise blocks larger than the configured buffer size
would pause indefinitely).

Conflicts:
	src/net.cpp
Fixes for blockchain corruption on OSX

As we'd previously learned, OSX's fsync is a data eating lie.

Since 0.8.4 we're still getting some reports of disk corruption on
 OSX but now all of it looks like the block files have gotten out of
 sync with the database. It turns out that we were still using fsync()
 on the block files, so this isn't surprising.