PLATFORM-292 memcache monitoring

Gather memcache operations statistics (**number of gets, sets, deletes, misses and hits**) and more detailed keys stats to get **the list of top hits and misses**.

Memcache **keys are normalized**:

* wiki-specific prefix is removed (replaced with ``*``)
* all IDs and hashes key parts (`[a-f0-9]+`) are replaced with ``*`` as well

Example stats packet:

```php
Array
   (
            [0] => Array
                (
                    [time] => 1418994389.3468
                    [event] => memcache_stats_counters
                    [data] => Array
                        (
                            [name] => get
                            [value] => 92
                        )

                )

            [1] => Array
                (
                    [time] => 1418994389.3469
                    [event] => memcache_stats_counters
                    [data] => Array
                        (
                            [name] => miss
                            [value] => 72
                        )

                )

            [2] => Array
                (
                    [time] => 1418994389.3471
                    [event] => memcache_stats_counters
                    [data] => Array
                        (
                            [name] => set
                            [value] => 73
                        )

                )

            [3] => Array
                (
                    [time] => 1418994389.3472
                    [event] => memcache_stats_counters
                    [data] => Array
                        (
                            [name] => add
                            [value] => 4
                        )

                )

            [4] => Array
                (
                    [time] => 1418994389.3473
                    [event] => memcache_stats_counters
                    [data] => Array
                        (
                            [name] => delete
                            [value] => 3
                        )

                )

            [5] => Array
                (
                    [time] => 1418994389.3474
                    [event] => memcache_stats_keys
                    [data] => Array
                        (
                            [bucket] => hits
                            [key] => WikiFactory::getCategories:*
                        )

                )

            [6] => Array
                (
                    [time] => 1418994389.3475
                    [event] => memcache_stats_keys
                    [data] => Array
                        (
                            [bucket] => hits
                            [key] => *:user:id:*
                        )

                )

            [7] => Array
                (
                    [time] => 1418994389.3475
                    [event] => memcache_stats_keys
                    [data] => Array
                        (
                            [bucket] => hits
                            [key] => wikicities:user_touched:*
                        )

                )
)
```

$wgMemcacheStatsSampling will be used to set a sampling rate of stats being sent to InfluxDB.

@wladekb / @jcellary / @michalroszka 
...and then here we don't need this entire "if".