Implement read/write with file offset change

[chenhao-ye]

The tricky piece of read/write (in contrast to pread and pwrite) is, changing file offset would imply a serialization point while OCC will pick another serialization point. Such divergence could result in anomalies.

The easiest way to resolve it is to use a lock for file offset. A multithreaded program that has concurrent read/write on a file usually uses pread and pwrite instead.

A better solution is to fully exploit OCC: in BlkTable.update, a thread A acquires the current offset and the last offset-changing thread's tx tail. Then it performs its IO, commits, and leaves the tx tail it has seen (this is the serialization point). Another thread B that does IO concurrently should see A's offset changing and A does not finish (probably implemented as a ticket lock etc); B could proceed and when it is about to commit, it must ensure A has already committed and it must ensure its' commit is after A.

[chenhao-ye]

Fixed in #143.