Make the UNUSUAL warning in plugin-bcli depend on rpcclienttimeout

If a user has set `rpcclienttimeout` to a higher value than the default 60 seconds, it seems logical that the corresponding UNUSUAL warning in `bcli_finished` when a rpc request takes longer than 10 seconds, should also increase proportionally.

This is how I've implemented it (in `bcli_finished`, `plugin-bcli`) but if this change is desired, I leave it to the developers to implement it in the proper manner.

```
u64 msec_unusual;

/* For default rpcclienttimeout=60, "msec_unusual" should equal 10 seconds -> 10000 msec */
msec_unusual=(u64) (bitcoind->rpcclienttimeout/6*1000);
if (msec > msec_unusual)
plugin_log(bcli->cmd->plugin, LOG_UNUSUAL,
"bitcoin-cli: finished %s (%"PRIu64" msec)",
bcli_args(tmpctx, bcli), msec);
```