Refactor Agent

[felipejfc]

With this pr we send writes from only 1 goroutine (fixes #112), also, make the logic more readable.

[felipejfc]

@cscatolini I'd like your review since you were the one who implemented jaeger and I made some changes to reporting hoping not to change the logic :)

[coveralls]

Pull Request Test Coverage Report for Build 1032

• 52 of 70 (74.29%) changed or added relevant lines in 3 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 69.122%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
metrics/report.go	1	3	33.33%
service/handler.go	2	5	40.0%
agent/agent.go	49	62	79.03%

Files with Coverage Reduction	New Missed Lines	%
agent/agent.go	4	85.47%
cluster/etcd_service_discovery.go	4	81.34%

Totals
Change from base Build 1029:	0.2%
Covered Lines:	4580
Relevant Lines:	6626

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💛 - Coveralls