Add tx fee to the db and track for webhooks

[vtnerd]

Someone has privately requested that webhooks include the transaction fee. This was never reported by the MyMonero API, so the LWS-db doesn't even track it. This contains the first in-place update to the DB to get the transaction fee working.

Limitations:

• Only ring-ct transactions are supported (Jan 2017). Older transactions will be reported as 0 for fee.
• A rescan is not performed of existing transactions in the DB, instead the fee is marked as 0. A rescan will correctly update the fee.
• The fee is not returned in the REST API, only the web hook API.