[FLINK-1244] setCombinable() returns operator

This closes #205
[FLINK-1172] [docs] Fix broken links in documentation

This closes #206
Fix flaky test SumMinMaxITCase
[FLINK-1246] Add additional debug output to flaky recovery test
[streaming] Source parallelism API update

The new preferred way is calling setParallelism() after adding the SourceFounction. This solution works any other operator.
[streaming] Fixed async buffer sending at end of AbstractInvokable