New module type for implementing app level session draining

[paulikaila-wl]

Introduces a new interface SessionModule. If a Module implements the SessionModule interface it will be treated differently when session draining is enabled.

When session draining starts, Pitaya will call StartSessionDraining() on all registered SessionModules. And when checking if sessions have been drained, Pitaya will call SessionCount() on all registered SessionModules to check they have all reached zero.

[felipejfc]

Thx for the contribution @paulikaila-wl! Do you mind documenting it? here: https://github.com/topfreegames/pitaya/blob/main/docs/features.md#modules

[paulikaila-wl]

I added some documentation.

[coveralls]

Pull Request Test Coverage Report for Build 8827576094

Details

• 27 of 66 (40.91%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 61.84%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app.go	25	36	69.44%
module.go	2	30	6.67%

Files with Coverage Reduction	New Missed Lines	%
groups/memory_group_service.go	1	84.25%

Totals
Change from base Build 8807700949:	-0.3%
Covered Lines:	4834
Relevant Lines:	7817

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💛 - Coveralls

[felipejfc]

LGTM