Force settle if balance exhausted

[vkuznecovas]

Fixes issues where those who had issues settling in the first place are not settling now.

[codecov-commenter]

Codecov Report

Merging #2306 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2306      +/-   ##
==========================================
- Coverage   48.09%   48.06%   -0.04%     
==========================================
  Files         276      276              
  Lines       12874    12890      +16     
==========================================
+ Hits         6192     6195       +3     
- Misses       6086     6097      +11     
- Partials      596      598       +2     

Impacted Files	Coverage Δ
session/pingpong/accountant_promise_settler.go	44.75% <0.00%> (-1.13%)	⬇️
session/pingpong/invoice_tracker.go	65.50% <0.00%> (-2.58%)	⬇️
core/policy/oracle.go	86.48% <0.00%> (-5.41%)	⬇️
nat/traversal/pinger.go	74.27% <0.00%> (ø)
p2p/channel.go	76.62% <0.00%> (+2.29%)	⬆️
tequilapi/endpoints/sse_handler.go	74.78% <0.00%> (+2.60%)	⬆️

---

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 e5bdc55...db0df7f. Read the comment docs.