Passive & Active Sync

[shawn-zil]

Introduce a new category of sync: passive-sync.

Active-sync is driven by Proposals i.e. when a received Proposal is missing a parent. This is the normal sync that happens when a new node is introduced, or when a node falls out of sync e.g. missed a Proposal. The purpose of active-sync is to allow the node to catch up to its leader.

Passive-sync is driven by Timeouts i.e. when there are no Proposals to process. This is the new form of sync. The purpose of passive-sync is for a node to fill out its history, all the way to Genesis, if so desired. Since it is not crucial to the real-time operation of the node, it should only be done in the background.

Passive-sync should not interfere with any active-sync i.e. no passive-sync should start if any active-sync is already in progress; and active-sync will interrupt any passive-sync in progress.