Replication: add REPLCONF CAPA EOF support.

Add the concept of slaves capabilities to Redis, the slave now presents
to the Redis master with a set of capabilities in the form:

    REPLCONF capa SOMECAPA capa OTHERCAPA ...

This has the effect of setting slave->slave_capa with the corresponding
SLAVE_CAPA macros that the master can test later to understand if it
the slave will understand certain formats and protocols of the
replication process. This makes it much simpler to introduce new
replication capabilities in the future in a way that don't break old
slaves or masters.

This patch was designed and implemented together with Oran Agra
(@oranagra).

src/server.h

@@ -299,6 +299,10 @@ typedef long long mstime_t; /* millisecond time type. */
 #define SLAVE_STATE_SEND_BULK 8 /* Sending RDB file to slave. */
 #define SLAVE_STATE_ONLINE 9 /* RDB file transmitted, sending just updates. */
 
+/* Slave capabilities. */
+#define SLAVE_CAPA_NONE 0
+#define SLAVE_CAPA_EOF (1<<0)   /* Can parse the RDB EOF streaming format. */
+
 /* Synchronous read timeout - slave side */
 #define CONFIG_REPL_SYNCIO_TIMEOUT 5
 
@@ -569,6 +573,7 @@ typedef struct client {
                                        should use. */
     char replrunid[CONFIG_RUN_ID_SIZE+1]; /* master run id if this is a master */
     int slave_listening_port; /* As configured with: SLAVECONF listening-port */
+    int slave_capa;         /* Slave capabilities: SLAVE_CAPA_* bitwise OR. */
     multiState mstate;      /* MULTI/EXEC state */
     int btype;              /* Type of blocking op if CLIENT_BLOCKED. */
     blockingState bpop;     /* blocking state */