Fix OPENDJ-916 - MemoryBackend in SDK should support size limit
Review CR-2386

Change is limited to MemoryBackend class :
* added size limit support in handleSearch() method, throwing error with ResultCode.SIZE_LIMIT_EXCEEDED if limit is exceeded
* refactored handleSearch to avoid duplicate code for SearchScope.SINGLE_LEVEL and SearchScope.WHOLE_SUBTREE, adding new private method searchWithSubordinates()

Updated test class MemoryBackendTestCase
* added tests for size limit for both SearchScope.SINGLE_LEVEL and SearchScope.WHOLE_SUBTREE
* added test to ensure that we can get all entries from the memory backend.