* Remove the double-link list for DDL callback function 
* Retire ResGroupOpts.
* Remove CommandCounterIncrement() in AlterResourceGroup as we no longer
query pg_resgroupcapability in the callback function.
* alter memory_spill_ratio don't wakeup slots
* acquire AccessExlusiveLock on pg_resgroupcapability in CreateResGroup
* Change interface of GetResGroupCapabilities
* update test cases
Signed-off-by: NNing Yu <nyu@pivotal.io>