Improve BGP load optimization HLD with ZMQ

[liuh-80]

According to discussion about improve BGP load optimization HLD with ZMQ, this PR will replace Orchagent improvement with ZMQ solution.

However, following part in HLD need update, because the reference content been remove and replaced with ZMQ:

### 2. Add an assistant thread to the monolithic Syncd main event loop workflow

For Syncd, we need to decouple _consumer.pop(kco, isInitViewMode())_ from _processSingleEvent(kco)_.

### Syncd [similar optimization to orchagent]

Similar case for syncd with orchagent. In our proposal, consumer.pop and processSingleEvent(kco) is decoupled.

[liuh-80]

This part need re-write with Syncd code example, because orchagent already improve with ZMQ.

[liuh-80]

@a114j0y please check and give your comments regarding to this HLD change proposal.