`SyncManager` fails to find the `lastFinishedSync` when using an async persistence adapter

[obedm503]

The SyncManager fails to find the lastFinishedSync when using an async persistence adapter when attempting to start sync soon after creation. This issue only happens with async persistence adapters that might take several ms loading data like an indexeddb persister and not with sync persisters like local storage.

const syncManager = new SyncManager({  
  persistenceAdapter: (id) => idbPersister(id),
  pull: async ({ name, lastFinishedSyncEnd }) => {
    // lastFinishedSyncEnd is always undefined on the first sync
    const response = await fetch(`/api/collections/${collectionOptions.name}?since=${lastFinishedSyncStart}`)
    return await response.json()
  },
  // ...
})

const collection = new Collection()
syncManager.addCollection(collection, {
  name: 'todos',
})

syncManager.sync('todos') // will always load all the data instead of just data since `lastFinishedSyncEnd`

My current workaround is to delay calling sync or syncAll until syncManager.syncOperations, syncManager.changes, syncManager.remoteChanges, and syncManager.snapshots have all emitted the persistence.pullCompleted event once. This is not ideal because I'm using private properties. Ideally, the sync manager would emit some type of ready or init event once its internal collections have all loaded into memory.

[maxnowack]

I've just opened #949 With this PR, the SyncManager waits until the persistence adapters of the internal collections are ready before the sync starts