IOVDbFolder: fill object cache when reading data from CREST

This change implements a mechanism for filling the IOVDbFolder cache with data read from CREST. The mechanism works similarly to what we get when reading data from COOL.

The current version does not yet support CREST folders with multi-iov payloads. The support for such folders will be added in future revisions.

Also added a new script to the DataModelRunTests package: CondReadWriteCrest.py. This is not yet the final version. Currently, it is testing only CREST folders that cross IOV boundaries during the job. Later we plan to add testing of the so-called extensible folders, i.e. the folders that get new IOVs while the job is running.

CC the ADAM team members: @gunal @formica @mavogel @nozturk @aleksand @mmineev