This is one of the more significant fields we print, but there's no
need to allocate a temp buffer or escape the resulting string.
MCP results from 5 runs, min-max(mean +/- stddev):
store_load_msec:34048-36002(35070.4+/-8.5e+02)
vsz_kb:2637488
store_rewrite_sec:35.110000-38.120000(36.604+/-1.2)
listnodes_sec:0.830000-1.020000(0.95+/-0.065)
listchannels_sec:48.560000-55.680000(52.642+/-2.7)
routing_sec:29.800000-33.170000(30.536+/-1.3)
peer_write_all_sec:49.260000-52.490000(50.316+/-1.1)
MCP notable changes from previous patch (>1 stddev):
-listchannels_sec:55.390000-58.110000(56.998+/-0.99)
+listchannels_sec:48.560000-55.680000(52.642+/-2.7)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>