feat!: implement sync protocol

[Gozala]

It got really messy, but here is a high level overview of the changes

• differential.js exports differentiate module that calculates a delta between local tree and remote tree that implements PullSource interface.
  • It also defines SyncSource and SyncTarget classes that adapt underlying tree reader and pull source to arguments expected by okra's sync function.
    • This is largely fallout of okra library not being completely aligned with it's own type defs.
    • Our implementations also use Task based interface in order to abstract sync / async sources.
• Service now optional takes Sync.Service that exposes synchronization protocol for POST requests with accept: application/okra-sync header.
• Sync protocol queues requests and processes them one at a time.
  • ℹ️ It is critical that db's aren't mutated during sync, otherwise one might encounter errors.
  • ⚠️ Either use sync protocol or legacy HTTP interface, using both would lead to errors as mutating db mid-sync will lead to problems.
  • 🏗️ We probably should split sync into push / pull jobs, because pulls could be paralleled and pushes need to be processed one at a time.
• 🏗️ We need to do some work to prevent writes to local db during sync also as this can also lead to problems