OPENDJ-2709 Reduce lock contention in SubentryManager for modify/delete requests

For modify operations return immediately if the operation does not
target a sub-entry (lock-free). For delete operations return immediately
if the targeted entry is not a sub-tree containing sub-entries (requires
shared lock). This change improves modify throughput by over 50% against
an in-memory backend.

In DITCacheMap: make DITSubtreeSet.isEmpty() O(1) instead of O(N).