OPENDJ-1716 Various PluggableBackend/Storage refactorings
Code review: Nicolas Capponi

PersistItStorage.java:
Removed remove(), superseded by delete() to go along with CRUD metaphor.
Changed update() to only update if the new value differs from the old value + returned a boolean indicating if the record was updated in the tree.

ReadableStorage.java:
Added javadocs.

UpdateFunction.java:
In update(), now return a boolean.

WriteableStorage.java:
Removed remove(), superseded by delete() to go along with CRUD metaphor.
Added javadocs.

DatabaseContainer.java:
Consequence of replacing WriteableStorage delete() by remove().