Optimized ottoArrayToStrings in ChannelMapper
_changes feed notices changes to channel access

* Wildcard channel list ["*"] works properly now.
* Changes feed checks channel access before every document check.
* If a new channel appears, it'll be returned starting from seq 0.
Now tracking seq#s at which users get added to channels.

NOTE: The user and role JSON in the admin REST API now represent channel
access lists as mappings from channel name to sequence number, not as
arrays of channel names. So you'll see something like:
  admin_channels: {"chan1":17, "chan2":1}
instead of
  admin_channels: ["chan1", "chan2"]
If you add channels to admin_channels after the database has been active,
use the database's current sequence number as the value to associate
with newly-added channel names.
(This will be cleaned up later!)