#1958 OMemoryWatchDog issue : WeakHashMap used for listeners

Proposed solution: the returned Listener by
OMemoryWatchDog.addListener() is the wrapper. The caller MUST reference
this returned wrapper with a strong ref.
Merge branch 'develop' of https://github.com/orientechnologies/orientdb into develop
#1958 OMemoryWatchDog issue : WeakHashMap used for listeners

cleanp debug
Merge pull request #1964 from scenari/develop

#1958 OMemoryWatchDog issue : WeakHashMap used for listeners