optimized to work from multiple threads. It has too many frequent
transaction from multiple threads, and that tends to result into the
'database is locked' error of sqlite.
With the new implemenation, we're using the caching mechanism, which
keep the data in the memory all the time, and saves it on request
completion, and loads it only for the first time. Also, it will storage
the data using pickle, which will be faster than accessing sqlite.
Fixes#1329
61698b7b4d