when making swap proposals, optionally add fractional fee output

[cbeck88]

this non-breaking change makes the swap proposals compatible with latest deqs revisions

• the GenerateSwapRequest gets an optional number of fee basis points and a fee address
• if fee basis points is nonzero, then it must be <= 10,000, and the fee address must be non null.
• an SCI output is added corresponding to this, based on this rate times the counter amount (fractional if this is partial fill and required if not)

additionally:

• Add a "GetKeyImages" API to mobilecoind. This is a simple, helpful utility.
• Set cq_count = 1 on the mobilecoind fog grpc environment. This was an oversight and may help mobilecoind spawn fewer threads.

Motivation

in deqs revision 55e0d4f121d7cb5c03e017c5027712dbe84feec7, the deqs node may be configured to require fractional outputs like this for quotes to be valid, in order that they may collect fees.

Future Work

none