fix issue with btMultiBody friction in combination with soft contacts (friction should not re-use normal contact cfm/erp)

implement friction anchors, position friction correction, disabled by default. Use colObj->setCollisionFlag(flag | CF_HAS_FRICTION_ANCHOR); See test/RobotClientAPI/SlopeFrictionMain.cpp. In URDF or SDF, add <friction_anchor/> in <contact> section of <link> to enable.
PhysicsServer: properly restore old activation state after releasing picked object
btMultiBodyConstraintSolver: disable flip/flop of contact/friction constraint solving by default (it breaks some internal flaky unit tests)

src/BulletCollision/NarrowPhaseCollision/btPersistentManifold.cpp

@@ -279,6 +279,7 @@ void btPersistentManifold::refreshContactPoints(const btTransform& trA,const btT
 			removeContactPoint(i);
 		} else
 		{
+            //todo: friction anchor may require the contact to be around a bit longer
 			//contact also becomes invalid when relative movement orthogonal to normal exceeds margin
 			projectedPoint = manifoldPoint.m_positionWorldOnA - manifoldPoint.m_normalWorldOnB * manifoldPoint.m_distance1;
 			projectedDifference = manifoldPoint.m_positionWorldOnB - projectedPoint;