refactor(cluster): rename clusters and loadbalances

[justxuewei]

What this PR does:

• rename clusters & loadbalances to enhance readability.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #1667 (afd6fc5) into 3.0 (9a12e99) will decrease coverage by 0.02%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##              3.0    #1667      +/-   ##
==========================================
- Coverage   41.26%   41.24%   -0.03%     
==========================================
  Files         254      254              
  Lines       14589    14589              
==========================================
- Hits         6020     6017       -3     
- Misses       7871     7873       +2     
- Partials      698      699       +1     

Impacted Files	Coverage Δ
cluster/cluster/base/cluster_invoker.go	24.17% <22.22%> (ø)
cluster/cluster/available/cluster.go	100.00% <100.00%> (ø)
cluster/cluster/available/cluster_invoker.go	73.33% <100.00%> (ø)
cluster/cluster/broadcast/cluster.go	100.00% <100.00%> (ø)
cluster/cluster/broadcast/cluster_invoker.go	80.00% <100.00%> (ø)
cluster/cluster/failback/cluster.go	100.00% <100.00%> (ø)
cluster/cluster/failback/cluster_invoker.go	75.82% <100.00%> (ø)
cluster/cluster/failfast/cluster.go	100.00% <100.00%> (ø)
cluster/cluster/failfast/cluster_invoker.go	71.42% <100.00%> (ø)
cluster/cluster/failover/cluster.go	100.00% <100.00%> (ø)
... and 12 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9a12e99...afd6fc5. Read the comment docs.