rpcserver: enforce keysend minimum carrier HTLC amount

[guggero]

Fixes an issue where attempting to pay a below-dust carrier HTLC satoshi amount would knock a channel offline.

Found after attempting the negative test mentioned here: lightninglabs/lightning-terminal#1089 (comment)
Nice catch, @GeorgeTsagk.

[GeorgeTsagk]

Looks good 🧮

[coveralls]

Pull Request Test Coverage Report for Build 15710061227

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 22 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+18.6%) to 55.856%

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Files with Coverage Reduction	New Missed Lines	%
address/mock.go	2	86.93%
asset/asset.go	2	80.06%
commitment/tap.go	2	84.77%
asset/mock.go	3	72.62%
tapchannel/aux_leaf_signer.go	3	43.43%
tapgarden/caretaker.go	4	78.15%
tapdb/multiverse.go	6	82.23%

Totals
Change from base Build 15693890710:	18.6%
Covered Lines:	53069
Relevant Lines:	95010

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💛 - Coveralls