"Fork block rejected" message is weird

@Insoleet had this message today, and he thought the received block was invalid.

However this message rather want to say "block of a minor fork branch, stored for later usage".

After investigating a bit the source code wrapping this logging instruction, it appears that the fork blocks are strangely recorded and do not trigger a SWITCH on fork block reception.

Investigating further...

So: I changed the way blocks are accepted by the node, and changed the message to "Fork block already known".

The old code was actually a bit weird, and the fork blocks were not accepted by the node. Instead, fork blocks were only pulled from the network. Consequently, fork resolution was a bit longer to occur because fork blocks were only retrieved during the pulling.

Now, new fork blocks are direclty registered and rememberded by each node, which locally sees the available branches and can switch whenever a branch matches the fork resolution rule.