fixed bgsave_in_progress in INFO when BGSAVEing with diskstore enabled, don't DEBUG FLUSHCACHE when bgsave is in progress.

implemented a different approach to IO scheduling, so object->storage is no longer used, instead there is a queue and hash table of IO tasks to process, and it is always possible to know what are the scheduled and acrtive IO operations against every single key.

fixed a few bugs in DS store, now it's somewhat about able to reply to 'GET foo' with a bogus value.

cron part of disk store object cache implemented. Objects are pushed as IO jobs if needed, so that the IO thread will process them.

touched key for WATCH refactored into a more general thing that can be used also for the cache system. Some more changes towards diskstore working.

SUBSTR is renamed to GETRANGE to have better consistency between command
names (with SETRANGE as its dual). GETRANGE is still aliased as SUBSTR.

Now the rdbSave* functions return the number of bytes written (or
required to write) in serializing a Redis object, writing to /dev/null
and using ftell (which doesn't work on FreeBSD) isn't needed anymore.

volatile-lru maxmemory policy segfault fixed, thanks to Anthony Lauzon for reporting the problem with the patch. Original patch modified a bit in order to avoid the double lookup if the policy is allkeys-lru

added noeviction policy to redis maxmemory. ZSCORE removed from the list of commands that can't be called when we are low on memory, this command was added in the past for a stupid error.

first attempt to non blocking implementation of slave replication and SYNC bulk data download. Never compiled so far...