server+lncfg: make max in-mem tasks configurable

Add a `MaxTasksInMemQueue` field to the `WtClient` config so that users
can change the default if they please.

sample-lnd.conf

@@ -1007,6 +1007,10 @@ litecoin.node=ltcd
 ; The maximum number of updates to include in a tower session.
 ; wtclient.max-updates=1024
 
+; The maximum number of back-up tasks that should be queued in memory before
+; overflowing to disk.
+; wtclient.max-tasks-in-mem-queue=2000
+
 [healthcheck]
 
 ; The number of times we should attempt to query our chain backend before