Reorganize code into core backend, spi interface, and implementations, as well as remove interface name ambiguity:

* rename ReadTxn to ReadableStorage
* rename ReadTransaction to ReadOperation
* rename WriteTxn to WritableStorage
* rename UpdateTransaction to WriteOperation.


git-svn-id: https://svn.forgerock.org/opendj/trunk@11439 41b1ffd8-f28e-4786-ab96-9950f0a78031