pyldb: add ldb.search_iterator()

pyldb: add ldb.search_iterator()

This is able to handle async requests, e.g. with a notification control
and processes results as they arrive instead of waiting for all results
before returning.

search_handle = ldb.search_iterator(...)

for e in search_handle:
    if not isinstance(msg, ldb.Message):
        # referral
        continue

    name = e["name"][0]

result = search_handle.result()

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>