[DefaultRouter] add a parallel bench test

[dferstay]

Motivation

The default router is called for every message published in the default configuration. Therefore, it must maintain a high level of performance in the face of code changes. Previously, we had a benchmark that measured serial routing performance, but we didn't have a way to measure performance of the router under parallel routing calls.

Modifications

Added a BenchmarkDefaultRouterParallel bench test case to default_router_bench_test.go module.

Verifying this change

I ran the default router bench test module by hand from the pulsar sub-directory:

$ go test  -bench="^BenchmarkDefaultRouterParallel$"  -count=10 -cpu=1,2,4,8,16 -run=^$

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): No
• The public API: No
• The schema: No
• The default values of configurations: No
• The wire protocol: No

Documentation

• Does this pull request introduce a new feature? No