Improve the scalability of SyncIoDisk

[tatetian]

The SyncIoDisk in sgx-disk is not scalable on concurrency.

The problem is that there is a lock hold while doing I/O. This lock can be removed if using read_at and write_at API.

[lucassong-mh]

Workaround: Use std::os::unix::fs::FileExt::read_at and write_at API, remove the lock.
Bench result:

Code:

[tatetian]

This is good perf improvement. But the implementation contains a bug... The offset should be calculated as follows.

offset += buf.as_slice().len()

Another thing is that the implementation should better use pwritev so that the number of OCalls is always one per request, instead of one per buffer.

[lucassong-mh]

Improvement: Use libc::ocall::preadv64andpwritev64 API.
Bench result:

Code:

[tatetian]

Great. Now we can submit a pull request.