[GSOC] Eliminate O(N^2) scaling from rw-conflict tracking in serializable transactions

Hi, all. There was a very strange phenomenon I couldn't explain. So I was wondering if you can help me.

I was trying to replace the linked list with a skip list in serializable transaction object for faster conflict tracking. But the performance is bad.
So I used the instruction "rdtsc" to compare the speed of my skip list and the original linked list. The skip list was about 1.5x faster.

The interesting thing is that if I added the instruction "rdstc" at the end of the function "RWConflictExists",
the performance of the whole system was increased by at most 3 times!
Here is the result.

| benchmarks | without rdtsc | with rdtsc |
| simpe read/write | 4.91 | 14.16 |
| ssibench | 9.72 | 10.24 |
| tpcb | 26.45 | 26.38 |

( The simple read/write benchmark has the most number of conflicts. )

The patch is attached. All the difference of the two columns is with/without a simple line of code:
__asm__ __volatile__ ("rdtsc");
But I don't know why this instruction will influence the performance so much!

BTW, after adding the "rdtsc" instruction, the performance is better than the original version about 10% at most.
That means, the skip list can work!

Looking forward to your advices.

--
Sincerely

Mengxing Liu