In this commit, we modify lnwire.MilliSatoshi to be an unsigned
integer. We do this as all values within the specification are meant to
be unsigned unless otherwise specified. Our usage of signed integers to
this date has caused some compatibility issues with the other
implementations, so this is the first step to reconciling these
compatibility issues.
This commit adds a new type to the lnwire package: MilliSatoshi. A
milli-satoshi is simply 1/1000th of a satoshi, and will be used for all
internal accounting when sending payments, calculating fees, updating
commitment state, etc. Two helper methods are added: ToBTC(), and
ToSatoshis() to make manipulation of the values easy.
