Resource group memory spill is similar to 'statement_mem' in
resource queue, the difference is memory spill is calculated
according to the memory quota of the resource group.

The related GUCs, variables and functions shared by both resource
queue and resource group are moved to the namespace resource manager.

Also codes of resource queue relating to memory policy are refactored in this commit.
Signed-off-by: NPengzhou Tang <ptang@pivotal.io>
Signed-off-by: NNing Yu <nyu@pivotal.io>