lightningd: avoid high memory usage spike at startup.

I saw this while watching top, and tracked it down.  We load all the
plugins to checksum them at startup:

```
$ ms_print massif.out.3312805 | head -n 50
--------------------------------------------------------------------------------
Command:            lightningd/lightningd.real --developer --log-level=trace --cltv-delta=6 --cltv-final=5 --watchtime-blocks=5 --rescan=1 --disable-dns --lightning-dir=/tmp/ltests-roazlc8h/test_xpay_fake_channeld_1/lightning-1/ --addr=127.0.0.1:46337 --allow-deprecated-apis=false --network=regtest --ignore-fee-limits=false --bitcoin-rpcuser=rpcuser --bitcoin-rpcpassword=rpcpass --bitcoin-datadir=/tmp/ltests-roazlc8h/test_xpay_fake_channeld_1/lightning-1/ --dev-fast-gossip --dev-bitcoind-poll=1 --log-file=- --log-file=/tmp/ltests-roazlc8h/test_xpay_fake_channeld_1/lightning-1/log --log-prefix=lightningd-1  --dev-fail-on-subdaemon-fail --dev-no-reconnect --autoconnect-seeker-peers=0 --subdaemon=channeld:../tests/plugins/channeld_fakenet --dev-throttle-gossip --grpc-port=37819 --dev-crash-after=3600 --bitcoin-rpcport=51623
Massif arguments:   (none)
ms_print arguments: massif.out.3312805
--------------------------------------------------------------------------------

    MB
446.5^#
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     |#    :: : ::@@:@:::::::::::::::::::::::::::@@::::::::::::::@::::@:::::::
   0 +----------------------------------------------------------------------->Gi
     0                                                                   75.66

Number of snapshots: 57
 Detailed snapshots: [1 (peak), 2, 10, 12, 33, 45, 49]

--------------------------------------------------------------------------------
  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
--------------------------------------------------------------------------------
  0              0                0                0             0            0
  1    295,677,530      468,189,872      447,087,069    21,102,803            0
95.49% (447,087,069B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->95.35% (446,440,727B) 0x2EFB99: tal_resize_ (tal.c:755)
| ->94.23% (441,180,860B) 0x2EBCD1: grab_fd (grab_file.c:45)
| | ->94.23% (441,180,860B) 0x2EBD54: grab_file (grab_file.c:63)
| |   ->94.23% (441,180,860B) 0x1A5CF7: file_checksum (plugin.c:315)
| |     ->94.23% (441,180,860B) 0x1A5EF1: plugin_register (plugin.c:355)
| |       ->94.23% (441,180,860B) 0x1AC62E: plugins_set_builtin_plugins_dir (plugin.c:2532)
| |         ->94.23% (441,180,860B) 0x16D614: find_subdaemons_and_plugins (lightningd.c:569)
| |           ->94.23% (441,180,860B) 0x16E9A1: main (lightningd.c:1226)
| |
| ->01.11% (5,219,417B) 0x2EBC32: grab_fd (grab_file.c:38)
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

lightningd/plugin.c

@@ -308,13 +308,17 @@ static void destroy_plugin(struct plugin *p)
 static u32 file_checksum(struct lightningd *ld, const char *path)
 {
 	char *content;
+	u32 crc;
 
 	if (ld->dev_no_plugin_checksum)
 		return 0;
 
 	content = grab_file(tmpctx, path);
-	if (content == NULL) return 0;
-	return crc32c(0, content, tal_count(content));
+	crc = crc32c(0, content, tal_count(content));
+	/* We could leave this around, but we checksum many files in a loop,
+	 * causing 450MB of allocations at startup! */
+	tal_free(content);
+	return crc;
 }
 
 struct plugin *plugin_register(struct plugins *plugins, const char* path TAKES,