Distinguish between calls and channels

[rostgaard]

The PBX (FreeSWTICH), as of now, uses the key-value (plain) but we could probably benefit from converting to JSON.
This will give us an easy way of forwarding information directly, and avoid a lot of data-conversion-dancing.

[rostgaard]

Updated the issue and put a deadline on it.

[rostgaard]

We have removed the JSON support from libesl due to serialization (object, not string) issues, so we are back to using old-fashioned key-value maps.