Change how ready_* fields on rethinkdb.table_status are grouped

[timmaxw]

Splitting this out from #3064:

We should also group the ready_* fields under a nested status field as follows:

{
    'status': {
      'ready_completely': True,
      'ready_for_outdated_reads': True,
      'ready_for_reads': True,
      'ready_for_writes': True,
  }
}

We should also rename ready_completely to all_replicas_ready.

[timmaxw]

I've implemented this and I'll put it up for review once CR 2285 is done.

[timmaxw]

This is up for review in CR 2293.

[timmaxw]

Merged into reql_admin in 5ef9e3b.