Avoid simultaneous RDB and AOF child process.
This patch, written in collaboration with Oran Agra (@oranagra) is a companion to 780a8b1d. 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.
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