Test added for db.remove()

Fix for https://bugs.launchpad.net/fuel/+bug/1401521 was committed w/o unit tests.
It is here.
StatSender is refactored slightly to make testing easier.

Closes-Bug: #1402000

Change-Id: Ida4120c612ba0ad3cfec18eb27d2e4a295d7c503
Merge "Test added for db.remove()"