LovefieldService = function() {
  // Following member variables are initialized within getDbConnection().
  this.db_ = null;
  this.si_ = null;
  this.hd_ = null;
  this.stockClosingPricesQuery_ = null;
  this.sectorClosingPricesQuery_ = null;

  window.ls = this;
}
A singleton service used by the rest of the application to make calls to the
Lovefield API.
@constructor