Diskquota extension needs two kinds of hooks:
1. hooks to detect active tables when tables are being modified.
2. hooks to cancel a query whose quota limit is reached.

These two kinds of hooks are described in detail in Wiki:
https://github.com/greenplum-db/gpdb/wiki/Greenplum-Diskquota-Design#design-of-diskquota
They are corresponding to two components: Quota Enforcement Operator
and Quota Change Detector.
Co-authored-by: NHaozhou Wang <hawang@pivotal.io>
Co-authored-by: NHao Wu <gfphoenix78@gmail.com>