Do not rebuild slots that didn't change wrt the last build

the implicit drop of unchanged slots require that we went through the MRs are resolved and that there is no explicit selection (to allow manual rebuilds)

This partially addresses #19 (closed) (in particular the part that makes LHCbNightlyConf!87 (closed) redundant).

What is definitely missing is:

• changes in data packages are not taken into account (not considered by --resolve-mrs)
• if the previous build is too old (artifacts removed) the build should be triggered in any case (unless we can pin the artifacts)
• we need a mechanism to whitelist slots so that they are always rebuilt, like the case of the slots based on lcg nightlies (this may be implemented in terms of slots metadata, see !257 (merged)