made disk_io_settings be synchronized through the job queue

docs/manual.rst

@@ -3193,6 +3193,7 @@ that will be sent to the tracker. The user-agent is a good way to identify your
 		bool use_parole_mode;
 		int cache_size;
 		int cache_expiry;
+		bool use_read_cache;
 		std::pair<int, int> outgoing_ports;
 		char peer_tos;
 
@@ -3420,6 +3421,9 @@ It defaults to 512 (= 8 MB).
 ``cache_expiry`` is the number of seconds from the last cached write to a piece
 in the write cache, to when it's forcefully flushed to disk. Default is 60 second.
 
+``use_read_cache``, is set to true (default), the disk cache is also used to
+cache pieces read from disk. Blocks for writing pieces takes presedence.
+
 ``outgoing_ports``, if set to something other than (0, 0) is a range of ports
 used to bind outgoing sockets to. This may be useful for users whose router
 allows them to assign QoS classes to traffic based on its local port. It is