trace: don't use randombytes_buf(), use pseudorand.

This is much faster to give 64 bits of data, and we don't need
cryptographic randomness.

This brings us back to 413ns per trace.

Before:
	real	0m5.819000-6.472000(6.2064+/-0.26)s
	user	0m3.779000-4.101000(3.956+/-0.12)s
	sys	0m2.040000-2.431000(2.2496+/-0.15)s

After:
	real	0m3.981000-4.247000(4.1276+/-0.11)s
	user	0m3.979000-4.245000(4.126+/-0.11)s
	sys	0m0.000000-0.002000(0.001+/-0.00063)s

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: lightingd: trimmed overhead of tracing infrastructure.

common/test/Makefile

@@ -118,6 +118,7 @@ common/test/run-splice_script:				\
 common/test/run-trace:					\
 	common/amount.o                                 \
 	common/memleak.o				\
+	common/pseudorand.o				\
 	common/trace.o					\
 	wire/fromwire.o					\
 	wire/towire.o