Include inbound-claimed-HTLCs in reported channel balances

Include inbound-claimed-HTLCs in reported channel balances

Given the balance is reported as "total balance if we went to chain
ignoring fees", it seems reasonable to include claimed HTLCs - if
we went to chain we'd get those funds, less on-chain fees. Further,
if we do not include them, its possible to have pending outbound
holding-cell HTLCs underflow the balance calculation, causing a
panic in debug mode, and bogus values in release.

This resolves a subtraction underflow bug found by the
`chanmon_consistency` fuzz target.