[Feature]: support configurable I/O space to drain at a time in edge-triggered mode

[panjf2000]

Description of new feature

Add a new option that specifies the number of bytes that gnet can read/write up to in one event loop of ET. For the moment, the default is 1MB.

Scenarios for new feature

It would be useful when users need to read/write more (or less) data in one event loop of ET.

Breaking changes or not?

No

Code snippets (optional)

type Options struct {
	...

	// EdgeTriggeredIOSpace specifies the number of bytes that `gnet` can 
	// read/write up to in one event loop of ET. The default is 1MB.
	EdgeTriggeredIOSpace int
}

Alternatives for new feature

None.

Additional context (optional)

None.