Move get_random_expr() into the database.
Move get_random_expr() into the database.

This increases reply count by about 2%.

wodim/cobe-ng,tiagochiavericosta/cobe,pteichman/cobe,LeMagnesium/cobe,DarkMio/cobe,LeMagnesium/cobe,pteichman/cobe,DarkMio/cobe,meska/cobe,meska/cobe,wodim/cobe-ng,tiagochiavericosta/cobe