Dependency Updates

• abseil-cpp 20200923 -> 20200923.2
  
• Protobuf 3.13.0 -> 3.14.0
  
• Add support for Gurobi 9.1.0
  
• drop Glog dependency (replaced by a custom implementation depending on abseil-cpp flags)
  
• drop Gflag dependency (replace by abseil-cpp flags component)
  

Bug Fixes

• Fix double counting of Gurobi floating license (#2227)
  
• Fix windows build (#2200)