Streams which truncated early with an EOF message would return a
"Failed to authenticate decrypted block" error. While technically
correct, this isn't a helpful error message as it masks the underlying
problem. This changes it to return "unexpected EOF" instead.
The rest of rclone knows it should retry such errors.
The corruption was caused when the file was read to the end thus
setting io.EOF and returning the buffers to the pool. Seek reset the
EOF and carried on using the buffers that had been returned to the
pool thus causing corruption when other goroutines claimed the buffers
simultaneously.
Fix by resetting the buffer pointers to nil when released and claiming
new ones when seek resets EOF. Also added locking for Read and Seek
which shouldn't be run concurrently.
This was caused by failing to reset the internal buffer on seek so old
data was read first before the new data.
The unit tests didn't detect this because they were reading to the end
of the file to check integrity and thus emptying the internal buffer.
Both code and unit tests were fixed up.
