[2.11.x] Fix OBR deadlock during group merge

https://issues.sonatype.org/browse/NEXUS-6708
http://bamboo.s/browse/NX-OSSF418

Deadlock was caused by allowing multiple merges of obr.xml as the same time. The fix will exclusively lock group obr.xml while performing the merge so other threads will not perform the merge.
Minor additional improvement: `createUid(request.getRequestPath())` could be replaced here by `obrUid`?