r13609: Get in the initial work on making ldb async

r13609: Get in the initial work on making ldb async
Currently only ldb_ildap is async, the plan
is to first make all backend support the async calls,
and then remove the sync functions from backends and
keep the only in the API.

Modules will need to be transformed along the way.

Simo
(This used to be commit 1e2c13b2d52de7c534493dd79a2c0596a3e8c1f5)