[net processing] Remove fClient and m_limited_node
fClient is replaced by CanServeBlocks(), and m_limited_node is replaced
by IsLimitedPeer().
https://github.com/bitcoin/bitcoin/pull/25514/commits/f65e83d51bfb6a34f1d5efccfb3d8786a51a4534
[net processing] Inline and simplify UpdatePreferredDownload
We inline UpdatePreferredDownload because it is only used in one
location during the version handshake. We simplify it by removing the
initial subtraction of state->fPreferredDownload from
nPreferredDownload. This is ok since the version handshake is only
called once per peer and state->fPreferredDownload will always be
false before the newly inlined code is called, making the subtraction a
noop.
https://github.com/bitcoin/bitcoin/pull/24543/commits/a4c55a93ef9277e1043c286120e2417652ee8bbb
This is a partial backport of core#25514 and core#24543
Depends on D14746