P2P: reply to new connections with a peer list even when reached maximum connections?

**User Story**
When a p2p node reached the "soft" maximum connections, new connections will be "softly" rejected after got the `hello` message. If all default public seed nodes reached the limit, new nodes will be unable to join the network, although it's rare, ideally we should have mechanisms to get around the situation.

More info: https://github.com/bitshares/bitshares-core/wiki/P2P-network-protocol .

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [x] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [x] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

**Additional Context (optional)**
Add any other context about your request here.

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start with the linked P2P network protocol wiki and trace the connection-handling path for hello messages and the soft maximum connection limit. Define the peer-list response behavior for newly connected peers when that limit is reached, then identify the corresponding test cases needed to verify that nodes can still discover peers.

Written by the indexing model from the issue text.