remove get_mailboxDN function from openchangedb interface

It's used only internally to make relantionships between folders and mailboxes
minor changes on mysql model file
added FIXMEs to functions not used anywhere

for now I won't unittest it, nor implemented in mysql backend
initial test for openchagedb_ldb

Not working yet, I have to add resources either a .ldb or .ldif file
create indexing context on the right memory context
cleaning up indexing test suite

 * Remove unnecesary typedefs
 * bad indexing_file format
 * Put global varibles as static
 * Remove initialize test, the check must be done in the setup function
 * Put indexing_backend_add_case function contents inside indexing_suite function
cleaning up test_suites, use static variables
the results need to have mailboxDN attribute
let ldb_init to create the tevent_context (related with returned ldb_ctx pointer)
create ldb_context on the right memory context
move test suites to subdirs