[IMPROVED] Unified user speed statistics

* [ADDED] getUserMiningStats to fetch all speed related data
 * Uses global cache if available, falls back to local query and then
   local caches
* [REMOVED] getUserHashrate, getUserSharerate, getUserAvgShareDifficulty
* [UPDATED] All code occurences for the above to use the new system

Probably won't change much since global cache was already used but does
help on the Dashboard since we combine at least 2 calls into one.
[FIX] Wait for global cache data, don't run queries
Merge pull request #1837 from MPOS/unified-user-miningstats

Unified user miningstats