Refactor and specific agent classes bootdelegation

[eyalkoren]

All classes except public API, OpenTracing bridge and some tests were moved and repackaged under an internal agent root - co.elastic.apm.agent. Bootdelegation was enabled for this root package.

closes #362

[codecov-io]

Codecov Report

Merging #376 into master will not change coverage.
The diff coverage is 83.33%.

@@            Coverage Diff            @@
##             master     #376   +/-   ##
=========================================
  Coverage     72.39%   72.39%           
  Complexity     1195     1195           
=========================================
  Files           131      131           
  Lines          4484     4484           
  Branches        459      459           
=========================================
  Hits           3246     3246           
  Misses         1029     1029           
  Partials        209      209

Impacted Files	Coverage Δ	Complexity Δ
...m/agent/plugin/api/TransactionInstrumentation.java	52.38% <ø> (ø)	3 <0> (?)
...agent/configuration/validation/RegexValidator.java	100% <ø> (ø)	6 <0> (?)
.../co/elastic/apm/agent/impl/payload/SystemInfo.java	55.55% <ø> (ø)	6 <0> (?)
...ic/apm/agent/servlet/ServletTransactionHelper.java	73.01% <ø> (ø)	38 <0> (?)
...va/co/elastic/apm/agent/impl/transaction/Http.java	73.68% <ø> (ø)	10 <0> (?)
...n/java/co/elastic/apm/agent/util/VersionUtils.java	60% <ø> (ø)	4 <0> (?)
...co/elastic/apm/agent/servlet/ServletApiAdvice.java	9.67% <ø> (ø)	2 <0> (?)
...elastic/apm/agent/impl/payload/ProcessFactory.java	79.59% <ø> (ø)	0 <0> (?)
...java/co/elastic/apm/agent/impl/transaction/Db.java	59.45% <ø> (ø)	12 <0> (?)
...ent/servlet/helper/StartAsyncAdviceHelperImpl.java	0% <ø> (ø)	0 <0> (?)
... and 121 more

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[felixbarny]

difficult to review but LGTM from skimming over it