Avoid simultaneous RDB and AOF child process.

This patch, written in collaboration with Oran Agra (@oranagra) is a companion
to 780a8b1. Together the two patches should avoid that the AOF and RDB saving
processes can be spawned at the same time. Previously conditions that
could lead to two saving processes at the same time were:

1. When AOF is enabled via CONFIG SET and an RDB saving process is
   already active.

2. When the SYNC command decides to start an RDB saving process ASAP in
   order to serve a new slave that cannot partially resynchronize (but
   only if we have a disk target for replication, for diskless
   replication there is not such a problem).

Condition "1" is not very severe but "2" can happen often and is
definitely good at degrading Redis performances in an unexpected way.

The two commits have the effect of always spawning RDB savings for
replication in replicationCron() instead of attempting to start an RDB
save synchronously. Moreover when a BGSAVE or AOF rewrite must be
performed, they are instead just postponed using flags that will try to
perform such operations ASAP.

Finally the BGSAVE command was modified in order to accept a SCHEDULE
option so that if an AOF rewrite is in progress, when this option is
given, the command no longer returns an error, but instead schedules an
RDB rewrite operation for when it will be possible to start it.

src/server.h

@@ -918,6 +918,7 @@ struct redisServer {
     time_t lastbgsave_try;          /* Unix time of last attempted bgsave */
     time_t rdb_save_time_last;      /* Time used by last RDB save run. */
     time_t rdb_save_time_start;     /* Current RDB save start time. */
+    int rdb_bgsave_scheduled;       /* BGSAVE when possible if true. */
     int rdb_child_type;             /* Type of save by active child. */
     int lastbgsave_status;          /* C_OK or C_ERR */
     int stop_writes_on_bgsave_err;  /* Don't allow writes if can't BGSAVE */