Prefix BOLT11 description with recipient name

[hsjoberg]

To provide better UX, transaction log should display the user which money was sent to.
As BOLT11 doesn't provide any way to a recipient name and node alias is not reliable or node could be hidden/unknown, it could be prefixed in the description of the BOLT11 invoice.

<name>: <description>

A name followed by a colon and two spaces could be used to clearly separate the name from the actual description. It should be easy to parse.

• Implement recipient name into description.
• Store known "identities" in sqlite and warn if two different pubkeys use the same name

TODO what to do when user does not provide a description

[hsjoberg]

Partly implemented in e0ab93f