Fix potential segfaults using freed memory.

Fix potential segfaults using freed memory.

In some code paths ltdb_context was still referenced even after we were returned
an error by one of the callbacks. Because the interface assumes that once an
error is returned the ldb_request may be freed, and because the ltdb_context was
allocated as a child of the request, this might cause access to freed memory.
Allocate the ltdb_context on ldb, and keep track of what's going on with the
request by adding a spy children on it. This way even if the request is freed
before the ltdb_callback is called, we will safely free the ctx and just quietly
return.