fix(pex): seed doesn't respond to pex requests

[lklimek]

Issue being fixed or feature implemented

On a devnet with > 10 masternodes, seed fails to advertise addresses in response to PexRequest.

This is caused by a combination of several factors:

1. When there is only 1 address in addressbook, seed limits number of returned addresses to 1. This causes PexResponse to contain only 1 address, belonging to the seed.
2. Timer which determines when the seed sends PexRequest is reset after processing each request. Multiple concurrent messages received on the Pex channel reset this timer before it is fired.

What was done?

1. Adjusted advertising logic to include self address when determining number of addresses to return.
2. Timer in pex channel processing is reset only after sending PexRequest
3. Refactored code to never access peerManage.store directly
4. Improved logging and error handling
5. Improved tests of peer manager advertising
6. Extraced some code from sendRequestForPeers to make critical section shorter
7. Refactored node/seed.go and node/node.go to decrease code redundancy.

How Has This Been Tested?

Tested on devnet, by providing a Docker image and ensuring the issue cannot be reproduced anymore.

Breaking Changes

None

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[shotonoff]

LGTM
left one question

[shotonoff]

approve