Revert "Remove AvailableBalances::balance_msat"

Revert "Remove AvailableBalances::balance_msat"

While removing the `balance_msat` field absolutely makes sense -
it is, at best, confusing - we really need a solid replacement for
it before we can do so. While one such replacement is in progress,
it is not complete and we'd like to not block our current release
on its completion.

This reverts commit ef5be580f51d6756612fea710516c0297203f4dc.