Add progressive batching for VRF/VXLAN device cleanup

[ypcisco]

What I did

• Implemented progressive batching for VRF and VXLAN device cleanup in vrfmgr and vxlanmgr constructors
• Batch size grows geometrically with 1.25x factor: 1→2→3→4→5→...→500 (max).
• Start small (batch=1) to prevent startup netlink congestion
• Devices deleted in parallel within each batch, batches executed sequentially

Why I did it

• Config reload/cold restart in scaled Vnet scenarios takes long time due to sequential synchronous deletion
• vrfmgrd and vxlanmgrd are blocked during cleanup, significantly delaying new device creation

How I verified it

• Created test_VRFMgr_ConstructorCleanup in test_vrf.py.
• Created test_vxlanmgr_constructor_cleanup in test_vxlan_tunnel.py.

[anish-n]

Just curious what is the value of doing it in increments, why don't we just set the batch size to some static number like 100 and do it in a batch like that? I see that we have stated there is startup netlink congestion if we batch too soon, have we seen performance issues if we batch too soon?