Threads blocking on Hibernate SequenceHiLoGenerator.generate() in EAP 6

Issue

A high number of threads are observed to be in a BLOCKED state with traces like the below

... waiting for monitor entry [0x00007f44b72ed000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at org.hibernate.id.SequenceHiLoGenerator.generate(SequenceHiLoGenerator.java:69)
        - waiting to lock <0x000000073bd65b08> (a org.hibernate.id.SequenceHiLoGenerator)
        at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:117)
        at org.hibernate.ejb.event.EJB3MergeEventListener.saveWithGeneratedId(EJB3MergeEventListener.java:71)
        at org.hibernate.event.internal.DefaultMergeEventListener.saveTransientEntity(DefaultMergeEventListener.java:265)
        at org.hibernate.event.internal.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:245)
        at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:183)
        at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:86)
        at org.hibernate.internal.SessionImpl.fireMerge(SessionImpl.java:833)
        at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:817)
        at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:821)
        at org.hibernate.ejb.AbstractEntityManagerImpl.merge(AbstractEntityManagerImpl.java:889)
        ...