Add /peers/count endpoint

[ghost]

It would be useful to get real count of all currently connected peers
/api/peers/count

[dakk]

I could implement it if main-dev tells that this call is really needed.

Propesed response format (same format as https://lisk.io/documentation?i=lisk-docs/APIReference#get-total-count-of-delegates):

{
  "success": true,
  "count": 26
}

[karmacoma]

@dakk Sure go ahead, I think would be useful 👍

[dakk]

I'm on it

[dakk]

@karmacoma in your opinion, is it necessary to count peers in db with connected=2 or I need to ask to express how many connections are active in the last period?
In a REST environment "connected peers" could mean everything

[karmacoma]

@dakk For now I think it would good enough to simply count by status i.e. 0,1,2.

Returning a result such as:

{banned: 1, disconnected: 0, connected: 100}