My test case is a mainnet gossip store with 22107 channels, and
time to do `lightning-cli listchannels`:
Before: `lightning-cli listchannels` DEVELOPER=0
real 0m1.396000-1.409000(1.4022+/-0.005)s
After:
real 0m1.307000-1.320000(1.3128+/-0.005)s
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>