Support Java 11 (7.x)

[zoliMolnar]

This pull request adds minimalistic support for Java 11.

[apeteri]

I also have some questions:

• Did any of the test projects break due to compile errors? I saw this with newer Eclipse releases where the test classpath is separate from regular classes.
• Does classgraph support PDE devMode classpaths now?

[apeteri]

In the Travis build, Tycho did not like JavaSE-11 as an execution environment value: https://travis-ci.org/b2ihealthcare/snow-owl/builds/469543966

[zoliMolnar]

@apeteri Neither of the test projects broke due to compilation errors.
ClassGraph supports it but currently it has a bug with the newer versions of eclipse (specifically versions from 2018-09). To resolve this I've forked ClassGraph, fixed the bug and opened a PR for it.

[cmark]

Please bump Tycho to v1.3 to be able to build projects with Java 11.
https://wiki.eclipse.org/Tycho/Release_Notes/1.3

[codecov-io]

Codecov Report

Merging #295 into 7.x will increase coverage by <.01%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##                7.x     #295      +/-   ##
============================================
+ Coverage     51.33%   51.33%   +<.01%     
- Complexity    10404    10414      +10     
============================================
  Files          1682     1681       -1     
  Lines         62099    61972     -127     
  Branches       6933     6934       +1     
============================================
- Hits          31876    31812      -64     
+ Misses        27492    27425      -67     
- Partials       2731     2735       +4

Impacted Files	Coverage Δ	Complexity Δ
...d/datastore/index/entry/SnomedConceptDocument.java	65.08% <ø> (ø)	16 <0> (ø)	⬇️
...ternational/snowowl/api/rest/SnowOwlApiConfig.java	100% <ø> (ø)	13 <0> (ø)	⬇️
...datastore/index/entry/SnomedComponentDocument.java	66.66% <ø> (ø)	4 <0> (ø)	⬇️
...om/b2international/snowowl/core/CoreActivator.java	100% <ø> (+61.9%)	4 <0> (ø)	⬇️
...core/internal/validation/ValidationThreadPool.java	89.47% <ø> (-0.53%)	2 <0> (ø)
...onal/snowowl/datastore/index/RevisionDocument.java	58.62% <ø> (ø)	4 <0> (ø)	⬇️
...2international/snowowl/fhir/api/FhirApiConfig.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...ore/index/constraint/SnomedConstraintDocument.java	69.59% <ø> (ø)	19 <0> (ø)	⬇️
...tore/index/entry/SnomedRefSetMemberIndexEntry.java	77.39% <ø> (ø)	70 <0> (ø)	⬇️
...store/index/entry/SnomedDescriptionIndexEntry.java	71.34% <ø> (+0.41%)	20 <0> (ø)	⬇️
... and 133 more

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