Module API for Forking

* create module API for forking child processes.
* refactor duplicate code around creating and tracking forks by AOF and RDB.
* child processes listen to SIGUSR1 and dies exitFromChild in order to
  eliminate a valgrind warning of unhandled signal.
* note that BGSAVE error reply has changed.

valgrind error is:
  Process terminating with default action of signal 10 (SIGUSR1)

1 files changed, 1 insertions, 4 deletions

diff --git a/src/db.c b/src/db.c
index 51f5a12b4..4a489036a 100644
--- a/src/db.c
+++ b/src/db.c
@@ -60,10 +60,7 @@ robj *lookupKey(redisDb *db, robj *key, int flags) {
         /* Update the access time for the ageing algorithm.
          * Don't do it if we have a saving child, as this will trigger
          * a copy on write madness. */
-        if (server.rdb_child_pid == -1 &&
-            server.aof_child_pid == -1 &&
-            !(flags & LOOKUP_NOTOUCH))
-        {
+        if (!hasForkChild() && !(flags & LOOKUP_NOTOUCH)){
             if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
                 updateLFU(val);
             } else {