In Redis RDB check: initial POC.

So far we used an external program (later executed within Redis) and
parser in order to check RDB files for correctness. This forces, at each
RDB format update, to have two copies of the same format implementation
that are hard to keep in sync. Morover the former RDB checker only
checked the very high-level format of the file, without actually trying
to load things in memory. Certain corruptions can only be handled by
really loading key-value pairs.

This first commit attempts to unify the Redis RDB loadig code with the
task of checking the RDB file for correctness. More work is needed but
it looks like a sounding direction so far.
