cli: send to several receivers in one command (#364)

[vsnation]

Fix: beam-wallet send can pay several receivers in one command (#364).

beam-wallet send --receivers "ADDR1:1.5,ADDR2:0.25" [--fee N] [--asset_id N] [--offline]
beam-wallet send --receivers_file payees.txt          # one "ADDR:AMOUNT" per line, '#' comments

Why / design: a Mimblewimble tx has one receiver, so there's no bitcoin-style single tx with many outputs. The command starts one tx per receiver in the same wallet session and waits until all of them finish (the CLI used to wait for exactly one). In the core, coin selection (CompleteBalance) and coin tagging (SaveCoins) run in the same synchronous step per tx, so two txs never take the same coins.

Checked before anything starts: entry format, amounts, duplicate receivers, address validity, options that can't be combined (--amount, -r, --utxo, --slatepack), and total amount + fees against the available balance.

How to see it (no node needed for the checks)

beam-wallet restore --pass=p --wallet_path=a.db --seed_phrase="abandon;abandon;abandon;abandon;abandon;abandon;abandon;abandon;abandon;abandon;abandon;about;"
beam-wallet send --pass=p --wallet_path=a.db -n 127.0.0.1:1 --receivers "<addr1>:1,<addr2>:2"
# master: unrecognised option '--receivers'

Improvement

input	master	this PR
--receivers A1:1,A2:2 (empty wallet)	unrecognised option '--receivers'	Not enough funds: 2 transactions need 3 BEAM plus 0.000002 BEAM in fees
--receivers_file payees.txt	unrecognised option	same check, 1.75 BEAM
--receivers A1:1,A1:2	–	duplicate receiver 'A1'
--receivers A1 / A1:0	–	invalid receiver entry …, expected <address>:<amount> / … Unable to send zero coins
--receivers A1:1 --amount 1	–	--receivers and --receivers_file can't be combined with --receiver_addr, --amount, --utxo or --slatepack
funded wallet	–	Transaction <txid> started: sending <amt> to <addr> per receiver; exits when all are done

Single send is unchanged. Two small side effects of sharing the amount parser (ParseAmount): amounts under 1 groth and nan are rejected (they used to become a 0-amount tx), and an invalid --amount error is now one line: the amount ('abc') is invalid. ….

Limits: with few big coins, change from the first tx isn't spendable until it confirms, so later txs can fail with "no inputs" even though the total balance is enough. Run split first in that case. Every SBBS receiver must come online, as with send. The txs aren't queued one after another, as suggested in the issue's comment; they start together.

Testing: new wallet_cli_test (list and amount parsing: file format, comments, malformed entries, zero, negative, 1,5, duplicates, sub-groth, nan, inf) passes; with the duplicate check disabled it fails 2 checks. New TestSendToManyReceivers (wallet_test, in-process test node) passes: one sender starts two txs back to back, both complete, each receiver gets its amount, and no coin is spent by both txs. The CLI rows above compare master and this branch. Fork CI (running): https://github.com/vsnation/beam/actions/runs/

Fixes #364