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

Hi, all. In the last week, I replaced linked list with hash table in SerializableXact.
I only replace inConflicts and outConflicts. The other linked lists, such as possibleUnsafeConflicts, I will modify them after other things work well.

There are still some bugs: the abort rate is much higher than before (from 11.3% to 72%). I'll figure out what's wrong in the next few days.

My design is as follow:

For hash table, key is the pointer of SerializableXact; Value is the RWConflictData object.
Hashcode is generated based on the SerializableXact pointer.
So, given a SerializableXact, we can quickly find if it is conflict with another SerializableXact.

Every SerializableXact has two tables: inConflictsTab and outConflictsTab.
They are allocated and initialized when creating PredXactList (in the function InitPredicateLocks).
When a SerializableXact object is released, it will release all its RWConflict objects, the hash tables are empty again.
Thus They can be reused directly after releasing.

NOTE: I stored RWConflictData in hash tables, instead of RWConflict object allocated by RWConflictPool.
After I remove other linked lists, the RWConflictPool can be omitted.

My code is on the :
https://github.com/liumx10/postgresql/commit/3fd9a7488de5ae19ce2ce19eae5f303153a079ff

There are 3 main modifications in summary:
1) Initializing hash table. Related functions: InitPredicateLocks

2) Setting, releasing and checking RWConflicts. Related functions: RWConflictExists, SetRWConflict, ReleaseRWConflict

3) There are some functions scanning all RWConflict in a SerializableXact.
Related functions: ReleasePredicateLocks, ReleaseOneSerializableXact, CheckForSerializableConflictOut,
CheckForSerializableConflictIn, OnConflict_CheckForSerializationFailure, PreCommit_CheckForSerializationFailure

Sincerely
--
Mengxing Liu