Support async OkHttp calls

[felixbarny]

closes #425

[codecov-io]

Codecov Report

Merging #526 into master will increase coverage by 1.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #526      +/-   ##
============================================
+ Coverage     64.19%   65.25%   +1.06%     
  Complexity       68       68              
============================================
  Files           173      180       +7     
  Lines          6466     7438     +972     
  Branches        746      950     +204     
============================================
+ Hits           4151     4854     +703     
- Misses         2063     2323     +260     
- Partials        252      261       +9

Impacted Files	Coverage Δ	Complexity Δ
...agent/plugin/api/ElasticApmApiInstrumentation.java	33.96% <0%> (-3.54%)	0% <0%> (ø)
...ava/co/elastic/apm/agent/impl/context/Request.java	70.06% <0%> (-3.01%)	0% <0%> (ø)
...co/elastic/apm/agent/servlet/ServletApiAdvice.java	5.95% <0%> (-1.63%)	0% <0%> (ø)
...astic/apm/agent/impl/transaction/AbstractSpan.java	90.41% <0%> (-0.33%)	0% <0%> (ø)
.../co/elastic/apm/agent/matcher/WildcardMatcher.java	95.15% <0%> (-0.14%)	0% <0%> (ø)
...tic/apm/agent/impl/context/TransactionContext.java	100% <0%> (ø)	0% <0%> (ø)	⬇️
...servlet/RequestStreamRecordingInstrumentation.java	47.61% <0%> (ø)	0% <0%> (?)
...gent/okhttp/OkHttp3ClientAsyncInstrumentation.java	21.31% <0%> (ø)	0% <0%> (?)
...vlet/ServletContextServiceNameInstrumentation.java	37.5% <0%> (ø)	0% <0%> (?)
...agent/okhttp/OkHttpClientAsyncInstrumentation.java	21.31% <0%> (ø)	0% <0%> (?)
... and 14 more

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[eyalkoren]

For some reason I made most comments on the version 2 instrumentation, but they apply to both

[eyalkoren]

Jenkins run the tests