Dead sockets returned from connection pool

Because of how the server (and possibly other intermediaries) work, a socket may be closed while it is idle without any protocol-level indication warning the client of this event.

The end result is that sometimes (particularly with views), a cached socket is retrieved, and at the first I/O operation, the operation fails because the socket has been closed, which in turn gives an error back to the user.

The library should attempt to check and see if the socket is closed first before returning it back to lcbcore.