Make json_start_member allocate extra space, which caller can directly
print into, and also make caller call js_written_some() itself.
MCP results from 5 runs, min-max(mean +/- stddev):
store_load_msec:35071-36817(35617.2+/-7e+02)
vsz_kb:2637488
store_rewrite_sec:35.790000-37.500000(36.6375+/-0.63)
listnodes_sec:0.690000-0.780000(0.72+/-0.035)
listchannels_sec:34.600000-36.340000(35.36+/-0.77)
routing_sec:30.310000-30.730000(30.445+/-0.17)
peer_write_all_sec:50.830000-52.750000(51.82+/-0.89)
MCP notable changes from previous patch (>1 stddev):
-listnodes_sec:0.720000-0.950000(0.86+/-0.077)
+listnodes_sec:0.690000-0.780000(0.72+/-0.035)
-listchannels_sec:40.300000-41.080000(40.668+/-0.29)
+listchannels_sec:34.600000-36.340000(35.36+/-0.77)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>