Move serialization to LogEvent.getBody().

[mikecdavis]

This PR moves the serialization from POJO to JSON into LogEvent#getBody(). This allows the serialization of the event to be offloaded to with the event handler when the HTTP request is built. This should be 100% backwards compatible with any existing EventHandler implementations. Initial benchmarks are much improved over the current master in tight loop simulations.

Stats after 100,000 calls to activate(...) in my simulation:

Metric (us)	PR-199	2.0.0-beta2	2.0.0-SNAPSHOT	THIS PR
Min	21	2	3	1
Median	29	2	9	6
Mean	41	3.8	10.2	6.1
75th Pct	40	3	11	7
95th Pct	85	7	18	11
99th Pct	126	26	43	21
999th Pct	713	65	237	232
Max	11825	17732	34384	12845

Note 2.0.0-beta2 was pre-batch endpoint, so isn't a valid comparison. Also performance runs can have some volatility, but the overall trends remain.

I've also wrapped some calls to logger.debug in logger.isDebugEnabled() to prevent unused log statement from triggering the serialization synchronously.

[coveralls]

Pull Request Test Coverage Report for Build 567

• 14 of 15 (93.33%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 89.373%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-api/src/main/java/com/optimizely/ab/event/LogEvent.java	6	7	85.71%

Totals
Change from base Build 566:	-0.03%
Covered Lines:	2338
Relevant Lines:	2616

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💛 - Coveralls

[aliabbasrizvi]

THANK YOU !! Taking a closer look now.

[aliabbasrizvi]

Awesome. LGTM.

[thomaszurkan-optimizely]

I had tried this approach but it turns out that the class loader and possibly some of the annotations evaluated at class loading time are actually taking up 60% of initial startup on first activate.

That said, I wanted to make this change anyway, as I would like to decouple json serialization implementation from core.

[ceimaj]

@thomaszurkan-optimizely can we get this into the 2.1 Android release?