We were trying to get the max capacity of a flow to see if we could add some
more sats, and hit an assertion:
tests/test_askrene.py:707:
```
DEBUG plugin-cln-askrene: notify msg info: Flow reduced to deliver 88070161msat not 90008000msat, because 107x1x0/1 has remaining capacity 88071042msat
DEBUG plugin-cln-askrene: notify msg info: Flow reduced to deliver 284138158msat not 284787000msat, because 108x1x0/1 has remaining capacity 284141000msat
**BROKEN** plugin-cln-askrene: Flow delivers 129565000msat but max only 56506138msat
INFO plugin-cln-askrene: Killing plugin: exited during normal operation
```
We need to *unreserve* our flow before asking for max capacity. We were
also missing a few less important cases where we altered flows without altering
the reservation, so fix those too.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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4b6a38fe0a