Fixed Double-Checked Locking by declaring fields to be volatile

[istsergey]

Double-Checked Locking without volatile field can work incorrectly
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

[gmessner]

@istsergey
While it may not harm marking these volatile, it is not needed as all the sub-API objects hold no state (they are immutable). To make them truly immutable, just need to mark the single field in AbstractApi final, as follows:

private final  GitLabApi gitLabApi;

[istsergey]

@gmessner you are right. Please reject my pull request.

[gmessner]

Closed per submitter's request.