Summary:
During the wangle accept process and in a few pieces of application code we transform one type of `AsyncSocket` to another, potentially *after* a write has occurred.

Since we do not currently carry `appBytesWritten` and `rawBytesWritten` during transformations, those values may not represent all bytes written. Likewise, if we transform from a `AsyncSSLSocket`, we lose count of the number of raw bytes written.

It's very difficult to reason about whether these problems will actually manifest, so I'd prefer to just guard against it.

With this change, we explicitly track the number of bytes written to the socket by incrementing a counter in `sendSocketMessage`, which is used by both `AsyncSocket` and `AsyncSSLSocket`. In addition, we copy the appBytesWritten and rawBytesWritten during socket moves.

Reviewed By: yfeldblum

Differential Revision: D24551958

fbshipit-source-id: 88416114b52931ff3ceef847401d556ccf0ab664