looprpc: fractional amount for static address loop-ins

The QuoteRequest adds a field auto_select_deposits to signal
that the specified quote amount should be coin-selected from
the client's deposits to determine the number of deposits to
quote for.
The StaticAddressLoopInRequest gets a new amount field that
either indicates that the swap amount should be allocated
from the passed deposits or coin-selected from all available
deposits.

Author: hieblmi