Add support for an LDIF backend.  Entries in this backend will be held in
memory, and all read operations will be served from memory, but the underlying
data will be stored in an LDIF file on disk and any write operation will cause
that LDIF file to be updated.

This backend supports all major operations, including moving/renaming non-leaf
entries with the modify DN operation, and also supports the subtree delete
control and LDIF import and export operations.  Backup and restore operations
are not currently supported.