Accelerate append write for cacheRemoteRevalidating gofer mount

Append write for cacheRemoteRevalidating gofer mount is a heavy operation, as it needs asking the gofer for file size firstly.

To accelerate it, we propose to leverage the non-posix behavisor to linux. According to man 2 pwrite:

```
POSIX requires that opening a file with the O_APPEND flag should have no effect on the
location at which pwrite() writes data. However, on Linux, if a file is opened with
O_APPEND, pwrite() appends data to the end of the file, regardless of the value of offset.
```

For VFS1, we can introduce an appendHandles aside already-existing readHandles and writeHandles.

But for VFS2, it seems that it's redesigned to keep only one handle for all operations.

Thoughts?