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peer: for RBF state machine block req if RBF iterating is outstanding

This fixes an issue in the itests in the restart case. We'd see an error
like:
```
2025-03-12 23:41:10.754 [ERR] PFSM state_machine.go:661: FSM(rbf_chan_closer(2f20725d9da1a5231dd58d6f6597066e0:0)): Unable to apply event err="invalid state transition: received *chancloser.SendOfferEvent while in ClosingNegotiation(local=LocalOfferSent(proposed_fee=0.00000193 BTC), remote=ClosePending(txid=072299154192ea16a7d6dd05e607b92ae, party=Remote, fee_rate=1 sat/vb))"
```

We resolve this by waiting to send in the new request unil the old one
has been completed.