Implement QueryWithIterator/SearchWithIterator

[verdie-g]

Closes #79

[yhmo]

@verdie-g
There is a conflict in this pr. Is it still in progress?

[verdie-g]

@yhmo there are few points I'm not sure about. For example, is the offset parameter a server-side parameter or it's a client-side hack? So it's ready to be reviewed. The conflicts should be easy to solve.

[yhmo]

@yhmo there are few points I'm not sure about. For example, is the offset parameter a server-side parameter or it's a client-side hack? So it's ready to be reviewed. The conflicts should be easy to solve.

SearchIterator doesn't support "offset" parameter.
QueryIterator can support "offset".

There is a seek() method to handle offset parameter:
In Java SDK: https://github.com/milvus-io/milvus-sdk-java/blob/303d8fe914ecde05d001e0f39d463ed13c74c9a8/sdk-core/src/main/java/io/milvus/orm/iterator/QueryIterator.java#L111
In CPP SDK: https://github.com/milvus-io/milvus-sdk-cpp/blob/64b2a5a5c97e7e04bb5732caf88f90a69b049bcc/src/impl/types/QueryIteratorImpl.cpp#L146

There is a limitation in milvus, query() interface returns no more than 16384 entities to client. Results of query() are ordered by primary keys.
If offset <= 16384, the seek() method calls query(limit=offset_value), the results are discarded, the last primary key value is recorded as the "current cursor position". The next() method will return results from this cursor position.
If offset > 16384, the seek() method will call query() for several times until the current cursor position is recorded.

It is much like a client-side hack.