1. move TMGXACT to PGPROC.
2. create distributed snapshot and create checkpoint will
traverse procArray and acquire ProcArrayLock. shmControlLock
is only used for serialize recoverTM().
3. get rid of shmGxactArray and maintain an array of TMGXACT_LOG
for recovery.

Author: Gang Xiong <gxiong@pivotal.io>
Author: Asim R P <apraveen@pivotal.io>
Author: Ashwin Agrawal <aagrawal@pivotal.io>