Pass slave's total resources to the ResourceEstimator and QoSController via Slave::usage().

For some triggers and sophisticated calculations in the Estimator and QoSController it is necessary to know the total slave's resources. These resources keeps changing due to dynamic reservations, so it cannot be passed during initialization. Thanks of https://issues.apache.org/jira/browse/MESOS-2818 it can be a part of ResourceUsage proto and be passed via Slave::usage()

This task includes:
1. Add 'total' to ResourceUsage (mesos.proto). There is TODO for that in code.
2. Fetch 'total' and attach it to usage in Slave::usage() method.