Construct a "compatibility struct" to convert `listpeerchannels`
output into legacy `listpeers` format.
Tests written using the legacy listpeers format can use the
`convert_legacy_listpeers` utility to construct a compatibility
struct.
The test_peerjudge_datagatherer malformed test needed to be malformed
differently to achieve the desired effect.
This bug was introduced after 0.11A, when we throttled InitialRebalancer
and made it far less aggressive.
The code keeping track of ongoing rebalances was not properly maintaining
a live pointer to the object, thus leading to a use-after-free bug.
