[#692] Restore partial import semantics for include/exclude branches

[vharseko]

Fixes #692

Problem

Since the import was rewritten around OnDiskMergeImporter (DJ 3.x), a partial import — import-ldif with --includeBranch and/or --excludeBranch — silently destroys all existing entries outside of the imported scope: the importer deletes the whole entry container (beforePhaseOne → entryContainer.delete()) before rebuilding it from the filtered LDIF content, and then aborts on the orphan check because the parent of the included branch is gone:

The import has been aborted because the entry '...ou=people' does not have a parent entry

The 2.x importer used to migrate those entries into the new container (MigrateExistingTask / MigrateExcludedTask in the old jeb/Importer.java, removed in OPENDJ-2337): --includeBranch defines the scope of the replacement, everything else in the base DN is preserved.

Found by running the slow TestNG group (excluded from CI), where TestImportAndExport.testImportExport fails on the include-branch: ou=people,dc=example,dc=com case with STOPPED_BY_ERROR.

Fix

Restore the 2.x behaviour in a way compatible with the on-disk-merge architecture (trees are rebuilt wholesale, so preserved entries must go through the import pipeline):

• Before the import starts, exportEntriesToPreserve() snapshots the entries which must survive into temporary LDIF files using the regular ExportJob machinery (the storage is opened READ_ONLY and closed again before startImport()): for each affected base DN, the entries outside of the include branches (unless the branches cover the whole base DN), plus the entries below exclude branches located inside the replaced scope.
• A new CompositeSource chains an unfiltered, non-schema-validating LDIFReaderSource over the snapshot files with the main LDIF source, so preserved entries are re-imported through the same phase-1/phase-2 pipeline.
• Containers not touched by the import (no include branch below them, or an entirely excluded base DN) are left alone, as before.
• Temporary files are deleted when the import finishes.

Verification

• TestImportAndExport: 12/12 pass — the include-branch case completes successfully and the base entry and sibling branches survive (no orphan abort). The test is moved out of the slow group into the default build.
• OnDiskMergeImporterTest: 29/29 pass.
• Backend regressions: JETestCase 32/32, PDBTestCase 32/32.