etcd: make max message size configurable

sample-lnd.conf

@@ -1169,6 +1169,9 @@ litecoin.node=ltcd
 ; testing with embedded etcd.
 ; db.etcd.embedded_log_file=/path/etcd.log
 
+; The maximum message size in bytes that we may send to etcd. Defaults to 32 MiB.
+; db.etcd.max_msg_size=33554432
+
 [postgres]
 ; Postgres connection string.
 ; db.postgres.dsn=postgres://lnd:lnd@localhost:45432/lnd?sslmode=disable