Poll Manager should be initialized lazily

[ItalyPaleAle]

Describe the bug

This package includes an init() method which is executed when the package is imported, even if not used:

netpoll/poll_manager.go

Lines 44 to 51 in 4b09897

	func init() {
	var loops = runtime.GOMAXPROCS(0)/20 + 1
	pollmanager = &manager{}
	pollmanager.SetLoadBalance(RoundRobin)
	pollmanager.SetNumLoops(loops)
	setLoggerOutput(os.Stderr)
	}

We import this package in Dapr to allow using Kitex as a binding. Using Kitex is optional, but because of this init() method allocating a poll manager, it causes additional goroutines (and more heap allocations) for every user.

To Reproduce

1. Import the package
2. Observe that a number of goroutines is automatically created

Expected behavior

Poll Manager should be initialized lazily, only when first needed.

[joway]

@ItalyPaleAle Thanks for reporting. I think it's make sense. We will implement lazy initialization in next version.

[joway]

#306