[IMPROVED] Cronbased global Hash-/Sharerate cache

* [ADDED] New statistic method to fetch all user mining stats
* [ADDED] New global cache to getUserHash/Sharerate calls
* [ADDED] New memcache key for new global cache

Addresses #1471 and may fix it already if no other changes are required.
[FIX] Honor target_bits for hashrate
[FIX] Removed debug output
[FIX] Also honor diff for share difficulties if unset
[REMOVED] Unused stats update
[FIX] Whoopsie SQL
[FIX] SQL again, sigh
Merge pull request #1472 from MPOS/issue-1471

Issue 1471