CNode has two members to keep protocol version:
nRecvVersion for received messages
nSendVersion for messages to send
After exchanging with VERSION and VERACK messages via protocol version
INIT_PROTO_VERSION, both nodes set nRecvVersion and nSendVersion to the
same value which is the greatest common protocol version.
This PR:
replaces two CNode members, nRecvVersion nSendVersion, with
m_greatest_common_version
removes duplicated getter and setter
There is no change in behavior on the P2P network.Backport of core PR17785.
Depends on D9193.