Refactor checksum upate

[jankowtf]

Currently, the checksum update takes place immediately when comparing the last stored checksum with the current checksum of referernces in class method $.compareChecksums():

self$.updateReferenceChecksum(
  ref = ref_uid, 
  checksum = ref_chk_current
)

As long as everything after the check goes well, this should not be a problem. But it could lead to inconsistencies if something fails. Hence, in order to make it "updated cycle safe" it would be better to place all updates that should only be carried out if everything worked out as planned to a on.exit() section