TCP sends block forever

[bboreham]

We set a deadline on reads but not on writes. This means if something is blocking the other end of the connection we will wait forever.

Since we expect connected peers to be fairly live, a deadline around the length of a heartbeat should allow us to error on stalled peers while not affecting normal operation.
Worst case, a connection would be torn down then re-established.

Seen at weaveworks/weave#3762