Refactored `QuotaTree` insertion logic.
Refactored `QuotaTree` insertion logic.

This patch replaces the `insert` method with `update` to make it
easier to update individual role's quota information in the tree.

Review: https://reviews.apache.org/r/71019

dforsyth/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,Gilbert88/mesos,chhsia0/mesos,abudnik/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos