Include inbound-claimed-HTLCs in reported channel balances

[TheBlueMatt]

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.

[codecov-commenter]

Codecov Report

Merging #1268 (1818c4a) into main (35d4ebb) will increase coverage by 1.55%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## main #1268 +/- ##
==========================================
+ Coverage 90.40% 91.96% +1.55% 
==========================================
Files 71 71 Lines 38135 47519 +9384 ==========================================
+ Hits 34476 43700 +9224 - Misses 3659 3819 +160 

Impacted Files	Coverage Δ
lightning/src/ln/channel.rs	92.95% <100.00%> (+3.60%)	⬆️
lightning/src/ln/functional_tests.rs	98.48% <100.00%> (+1.11%)	⬆️
lightning/src/util/enforcing_trait_impls.rs	86.82% <0.00%> (-2.66%)	⬇️
lightning-invoice/src/de.rs	79.55% <0.00%> (-1.18%)	⬇️
lightning/src/ln/chanmon_update_fail_tests.rs	97.73% <0.00%> (+0.03%)	⬆️
lightning/src/util/test_utils.rs	82.39% <0.00%> (+0.18%)	⬆️
lightning/src/chain/channelmonitor.rs	91.80% <0.00%> (+0.23%)	⬆️
lightning/src/routing/scoring.rs	94.31% <0.00%> (+1.04%)	⬆️
lightning/src/routing/router.rs	93.01% <0.00%> (+1.20%)	⬆️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 35d4ebb...1818c4a. Read the comment docs.

[TheBlueMatt]

Added a test, while adding it I discovered this is even harder to hit than I thought, you have to write a custom router on top and can only happen on the first HTLC after open cause otherwise the reserve value protects us against underflow. Would still be nice to land for 105, but its really hard to hit.

[arik-so]

looks great