Force node to always be in syncing mode

[SWvheerden]

Currently, nodes decide on their status on received ping and pong messages. These messages contain the sender's ChainMetaData.

Currently, its possible to change increase the accumulated_difficulty field, and then broadcast this out to every connected node. The following then happens:

• The connected node then thinks it needs to sync to you.
• It searches for a fork and "finds" on 0 blocks back.
• It thinks you are both in sync, and exit out of syncing mode

By the current defaults of 10_000 messages/ 100s, it means you can force the node into syncing mode every 10ms. Doing this whole dance takes longer than that.
The older flood of 1000 messages /100s means you can send one every 100ms which is also not sufficient.

[sdbondi]

Closed in #3478