Handle local retries

OpenSSL can return SSL_ERROR_WANT_READ even on blocking sockets after
renegotiation or heartbeats. Heartbeats ignore the flag that normally
makes these things handled internally anyway on 1.0.2. To handle this
more properly, we now have a special error type we use to signal this
event. The `Read` and `Write` implementation automatically retry in this
situation since that's what you normally want. People can use `ssl_read`
and `ssl_write` if they want the lower level control.

Closes #760