Type mismatch errors are now prefixed with WRONGTYPE.

So instead to reply with a generic error like:

-ERR ... wrong kind of value ...

now it replies with:

-WRONGTYPE ... wrong kind of value ...

This makes this particular error easy to check without resorting to
(fragile) pattern matching of the error string (however the error string
used to be consistent already).

Client libraries should return a specific exeption type for this error.

Most of the commit is about fixing unit tests.
REPLACE option for RESTORE.

The REPLACE option deletes an existing key with the same name (if any)
and materializes the new one. The default behavior without RESTORE is to
return an error if a key already exists.
COPY and REPLACE options for MIGRATE.

With COPY now MIGRATE does not remove the key from the source instance.
With REPLACE it uses RESTORE REPLACE on the target host so that even if
the key already eixsts in the target instance it will be overwritten.

The options can be used together.
BSD license added to every C source and header file.
Make clear that contributing code to the Redis project means to release it under the terms of the BSD license.
Copyright date fixed in COPYING file.
MIGRATE TCP connections caching.

By caching TCP connections used by MIGRATE to chat with other Redis
instances a 5x performance improvement was measured with
redis-benchmark against small keys.

This can dramatically speedup cluster resharding and other processes
where an high load of MIGRATE commands are used.
MIGRATE timeout should be in milliseconds.

While it is documented that the MIGRATE timeout is in milliseconds, it
was in seconds instead. This commit fixes the problem.
MIGRATE count of cached sockets in INFO output.
MIGRATE: fix default timeout to 1000 milliseconds.

When a timeout <= 0 is provided we set a default timeout of 1 second.
It was set to 1 millisecond for an error resulting from a recent change.
TTL API change: TTL returns -2 for non existing keys.

The previous behavior was to return -1 if:

1) Existing key but without an expire set.
2) Non existing key.

Now the second case is handled in a different, and TTL will return -2
if the key does not exist at all.

PTTL follows the same behavior as well.
Test: check if MIGRATE is caching connections.
MIGRATE: retry one time on I/O error.

Now that we cache connections, a retry attempt makes sure that the
operation don't fail just because there is an existing connection error
on the socket, like the other end closing the connection.

Unfortunately this condition is not detectable using
getsockopt(SO_ERROR), so the only option left is to retry.

We don't retry on timeouts.
Test: more MIGRATE tests.
Merge branch 'migrate-cache' into unstable
zmalloc: kill unused __size parameter in update_zmalloc_stat_alloc() macro.
zmalloc_get_private_dirty() function added (Linux only).

For non Linux systmes it just returns 0.

This function is useful to estimate copy-on-write because of childs
saving stuff on disk.
Children creating AOF or RDB files now report memory used by COW.

Finally Redis is able to report the amount of memory used by
copy-on-write while saving an RDB or writing an AOF file in background.

Note that this information is currently only logged (at NOTICE level)
and not shown in INFO because this is less trivial (but surely doable
with some minor form of interprocess communication).

The reason we can't capture this information on the parent before we
call wait3() is that the Linux kernel will release the child memory
ASAP, and only retain the minimal state for the process that is useful
to report the child termination to the parent.

The COW size is obtained by summing all the Private_Dirty fields found
in the "smap" file inside the proc filesystem for the process.

All this is Linux specific and is not available on other systems.
Merge branch 'unstable' of git://github.com/antirez/redis into unstable