Refactor Subnet lock for SubnetPort creation

[yanjunz97]

Instead of using lock for Subnet, this PR adds a cache in SubnetPort service
to record the number of SubnetPorts on each Subnet.
Read/Write operation for this cache is protected by lock to avoid race condition
and ensure the number of SubnetPort on the Subnet will be controlled below
the Subnet capacity.

Testing done:

• Create 16 Pods on default SubnetSet with size 16 (capacity 12), got all Pods ready;
  delete all Pods and observe the Subnets under the SubnetSet will be deleted by GC.
• Create 16 VMs on DHCPServer SubnetSet with size 16 (capacity 12), all Vms can get IP;
  delete all VMs and observe the Subnet under the SubnetSet will be deleted by GC.
• Create 16 VMs on DHCPServer Subnet with size 16 (capacity 12), only 12 VMs can PowerOn
  and get IP. The remainings will have VirtualMachineNetworkReady as NotReady and
  Message like network interface "eth0" error: subnetPort is not ready: SubnetPortNotReady - error occurred while processing the SubnetPort CR. Error: no valid IP in Subnet /orgs/default/projects/project-quality/vpcs/ns-1_7fcad11c-da38-4137-96ec-f0f80ae8d96b/subnets/subnet-test-1_60ebfa36-bbe2-4e6e-85df-8228be4e190a

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 28 lines in your changes missing coverage. Please review.

Project coverage is 73.29%. Comparing base (86c0d65) to head (8494fed).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/nsx/services/subnet/subnet.go	37.50%	9 Missing and 1 partial ⚠️
...kg/controllers/subnetport/subnetport_controller.go	41.66%	5 Missing and 2 partials ⚠️
pkg/nsx/services/subnetport/subnetport.go	91.66%	4 Missing and 1 partial ⚠️
pkg/controllers/common/utils.go	72.72%	2 Missing and 1 partial ⚠️
pkg/controllers/subnetset/subnetset_controller.go	82.35%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #974      +/-   ##
==========================================
- Coverage   73.37%   73.29%   -0.08%     
==========================================
  Files         118      118              
  Lines       16394    16397       +3     
==========================================
- Hits        12029    12019      -10     
- Misses       3581     3591      +10     
- Partials      784      787       +3     

Flag	Coverage Δ
unit-tests	73.29% <77.77%> (-0.08%)	⬇️

Files with missing lines	Coverage Δ
pkg/controllers/pod/pod_controller.go	61.57% <100.00%> (-0.41%)	⬇️
pkg/controllers/subnet/subnet_controller.go	63.74% <100.00%> (+0.10%)	⬆️
pkg/nsx/services/subnet/store.go	71.64% <ø> (-3.90%)	⬇️
pkg/nsx/services/subnetport/store.go	77.10% <ø> (ø)
pkg/controllers/common/utils.go	88.62% <72.72%> (-1.86%)	⬇️
pkg/controllers/subnetset/subnetset_controller.go	66.08% <82.35%> (+0.09%)	⬆️
pkg/nsx/services/subnetport/subnetport.go	82.08% <91.66%> (+1.25%)	⬆️
...kg/controllers/subnetport/subnetport_controller.go	74.74% <41.66%> (-0.95%)	⬇️
pkg/nsx/services/subnet/subnet.go	64.78% <37.50%> (-1.97%)	⬇️

... and 3 files with indirect coverage changes

[wenyingd]

LGTM

[lxiaopei]

a general comment: for the new introduced functions, could you add some docs to explain what the func is for? including the struct, import fields like "dirtyCounts" etc, it would be helpful for reviewer and other members to understand.