Support a seek API on the iterator to seek forward to a given key

Requested from Marty:

It would be nice to have a method on the iterator to seek forward.

Something like:

fdb_iterator_seek(fdb_iterator *iterator,
const void *seek_key,
size_t start_keylen);

The semantics would be to efficiently jump ahead on the iterator to the seek_key.
If the seek_key does not exist, it is the first key that sorts after the seek_key.

This functionality is useful when I have multiple iterators, and based on the data returned by one iterator I know I want to skip ahead on the other iterator to another key further ahead.