Asymetrical handshake based on connection direction

[niklaslong]

As part of our recent Ziggurat work at Equilibrium, we've retested Zcashd v4.7.0 and this was initially reported as a potential security vulnerability but was deemed safe for public disclosure.

This issue is related to #5168. The messages sent and received before the handshake is complete, differ based on the connection direction.

When the node initiates:

Ziggurat <-> Zcashd

<- version 
-> version 
<- verack
<- getaddr 
<- ping
<- getheaders 
-> verack

When the node receives the connection:

Ziggurat <-> Zcashd

-> version 
<- version 
<- verack
<- ping
<- getheaders 
-> verack

In both cases, ping and getheaders are sent by Zcashd before recieving the verack . In addition, getaddr is sent and requires a response. This makes the handshake asymmetrical and a response must be sent for any further communication to occur.

[niklaslong]

Update: reconfirmed in v5.0.0.