WIP: Feature/indexed deltas, attach index to delta messages to indicate order

[ajuvercr]

Extend delta message format to:

{
  "changeset": {
    "allowed_groups": ... ,
    "origin": ... ,
    "insert": [].
    "index": number
  },
}

The index is initialized from epoch in milliseconds and incremented with every delta message.
This way all delta messages are indexed monotonically increasing.

[ajuvercr]

This also coalesces some delta messages together in a bigger changeset, sepearte for different mu_call_id_trail.

[ajuvercr]

Let me just close this one, see #7