common: increase jsonrpc_io buffer size temporarily to aggrevate perf issues.

A client can do this by sending a large request, so this allows us to see what
happens if they do that, even though 1MB (2MB buffer) is more than we need.

This drives our performance through the floor: see next patch which gets
us back on track.

tests/test_coinmoves.py::test_generate_coinmoves (2,000,000, sqlite3):
	Time (from start to end of l2 node):	 271 seconds **WAS 135**
	Worst latency:				 105 seconds **WAS 12.1**

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/jsonrpc_io.c

@@ -8,7 +8,7 @@
 #include <errno.h>
 #include <unistd.h>
 
-#define READ_CHUNKSIZE 2048
+#define READ_CHUNKSIZE (1024*1024)
 
 struct jsonrpc_io {
 	MEMBUF(char) membuf;