Make startListeningOnPort() asynchronous

[hechaoli]

Call addListener on server channel's close future instead of blocking on it.

[vmwclabot]

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• Commit acad8f235d65f8507d8ab477a946c2f47597c792 must be signed by hechaol@outlook.com

[coveralls]

Pull Request Test Coverage Report for Build 5

• 6 of 7 (85.71%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 72.691%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ovsdb-client/src/main/java/com/vmware/ovsdb/service/impl/OvsdbPassiveConnectionListenerImpl.java	6	7	85.71%
<!--	Total:	6	7	85.71%	-->

Files with Coverage Reduction	New Missed Lines	%
ovsdb-client/src/main/java/com/vmware/ovsdb/netty/OvsdbClientHandler.java	2	69.47%
ovsdb-client/src/main/java/com/vmware/ovsdb/service/impl/OvsdbPassiveConnectionListenerImpl.java	2	81.82%
<!--	Total:	4		-->

Totals
Change from base Build 3:	-0.2%
Covered Lines:	1291
Relevant Lines:	1776

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💛 - Coveralls